In the real world, everyone dies eventually. In The Sims 4, however, that's not the case at all. While most of the digital people who inhabit your Sim worlds will pass on to the afterlife, there are actually several different methods for extending their lifespans indefinitely. Most of those methods have been added with paid expansion packs you'll need to buy in order to try them, but there are some free, base game options as well. If you like a particular Sim, it's possible to have them live forever--you'll just have to put in some work. Let's take a look at what it takes to live forever.
Before we get into the specific methods of gaining immortality, it feels prudent to point out two methods of duplicating potions and serums. Making enough Age-Away serums to live forever would take a long time and be very boring, and you'd have to regularly harvest the ingredients.
There are two ways to speed this process up. First is the Copypasto spell, which spellcasters can use to duplicate any object, and second is the cloning machine, which Sims in the Scientist career will have to build as part of their job duties. Either of these methods can be used to duplicate a Potion of Youth, an Age-Away Serum, or any individual ingredients. You'll need either the Realm of Magic (for Copypasto) or the Get To Work (cloning machine) expansions for these methods, however, and the base game doesn't have any similar methods of duplication.
The simplest way for a Sim to extend their life is to purchase the Potion of Youth from the Aspiration rewards store. For 1500 reward points, you'll be granted this potion that resets your Sim's current age--a standard adult Sim will be put back at 0 of 42 days lived in their adult stage after they drink a Potion of Youth. It's not particularly feasible to purchase a ton of these, since there are a lot of other useful things to spend your Sim's reward points on. But the cloning machine and Copypasto spell do work with the Potion of Youth.
The cowplant is basically the Sims version of Audrey II, the carnivorous plant from Little Shop of Horrors, and it's just as deadly--Sims who wander by it while it's hungry are likely to be eaten. When a cowplant murders a Sim, another Sim can obtain a potion called Essence of Life--which adds a few days to the Sim's lifespan each time they drink it. It would be tough for a Sim to live forever with this method, since it requires feeding the cowplant regularly, but it's possible.
Only scientist Sims can learn how to make the Age-Away Serum.
Like the Potion of Youth and Ambrosia, the Age-Away serum resets your Sim's current age back to zero. To obtain this recipe, you'll need your Sim to reach the maximum promotion in the Scientist career, and they'll eventually learn it by having a breakthrough at work.
The Age-Away serum requires alien ingredients that can only be found on Sixam, a planet that can only be accessed via rocket ship or using the Electroflux Wormhole Generator--both of which you will learn to build at work. You'll need Quill Fruit, which can be harvested on Sixam during the summer, any two alien crystals, and some xenopetrium. While the first two ingredients are easy to figure out, the xenopetrium is tougher.
This element is found in metals and crystals from Sixam, but you're extremely unlikely to be able to find it by analyzing those items yourself. Instead, send your space rocks to the Geo Council by clicking on them in your Sim's inventory, and they'll send back a pile of elements, almost certainly including xenopetrium.
The magic cauldron can help your Sim become an ancient, immortal wizard.
Just like the Potion of Youth and the Age-Away serum, the Potion of Rejuvenation resets the timer on your Sim's current age. The ingredients for this potion are valerian root, alabaster, and lemon, and since it's a magical potion, it will have to be made in a cauldron by a spellcaster.
While alabaster and lemon are common and easy to obtain, valerian root is primarily found near the greenhouse in the Magic Realm--and you'll also get lots of random free ingredients from experimenting with the cauldron to learn new recipes, so there's a decent chance you'll get enough ingredients for one or two of these potions by the time you learn how to make it.
You can also buy all magical ingredients from the shop vendors in the Magic Realm.
This magic potion works differently from the ones we've previously discussed. It doesn't stop your Sim aging, but rather it makes them never die from old age--if you want to play as an ancient wizard, this is the path to take. It requires mandrake, sage, morcubus, and a diamond. Mandrake and sage are fairly common plants found all over, and morcubus and diamonds can be found by digging up rock piles anywhere in the world. And like the other potion ingredients, you can get all of these from the shop vendors in the Magic Realm
Vampires can both live forever
While a vampire can die in an accident, they'll never age, meaning that simply being turned into a vampire will make your Sim immortal. Your Sim will still need to meet a vampire and convince them to turn them, but that's not too difficult. Vampires have one particular edge over the other forms of immortality, too: the ability to numb their emotions so they don't get upset for two days every time a member of the family tree dies.
Like vampires, werewolves can live forever--but it's not an inherent part of being a werewolf like it is with vampires. Instead, there's a perk at the end of the Werewolf skill tree, Immortal Werewolf, that will stop a werewolf from aging.
Sims can use the Flower Arranging Table to, you guessed it, create flower arrangements, and excellent-quality arrangements can be scented for extra benefits. A Sim that has maxed out their flower-arranging skill can use a very rare orchid to scent an arrangement, and doing so will give the arrangement similar properties to the Potion of Youth--when your Sim gives it a whiff, they'll get a little bit younger, and may have their current age reset to zero. Orchids are pretty hard to find, but you can get one by grafting a lily onto a snapdragon plant (or vice versa). The resulting plant will occasionally produce orchids.
Those who have the Snowy Retreat expansion, however, will find a single orchid plant growing at the peak of Mt. Komorebi. Your Sim will need to level up their rock climbing skill to at least 5 or 6 in order to make it through the expedition, but this is the fastest way of obtaining a guaranteed orchid.
The Mother Plant is the end boss in the StrangeVille expansion, but your Sim can become friends with it if they try hard enough. Sims who are friends with the Mother Plant can then ask it for a boon once each day--with one of the potential options being that the Mother Plant will extend their lives.
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Copypasto and cloning
Before we get into the specific methods of gaining immortality, it feels prudent to point out two methods of duplicating potions and serums. Making enough Age-Away serums to live forever would take a long time and be very boring, and you'd have to regularly harvest the ingredients.
There are two ways to speed this process up. First is the Copypasto spell, which spellcasters can use to duplicate any object, and second is the cloning machine, which Sims in the Scientist career will have to build as part of their job duties. Either of these methods can be used to duplicate a Potion of Youth, an Age-Away Serum, or any individual ingredients. You'll need either the Realm of Magic (for Copypasto) or the Get To Work (cloning machine) expansions for these methods, however, and the base game doesn't have any similar methods of duplication.
Potion of Youth (base game)
The simplest way for a Sim to extend their life is to purchase the Potion of Youth from the Aspiration rewards store. For 1500 reward points, you'll be granted this potion that resets your Sim's current age--a standard adult Sim will be put back at 0 of 42 days lived in their adult stage after they drink a Potion of Youth. It's not particularly feasible to purchase a ton of these, since there are a lot of other useful things to spend your Sim's reward points on. But the cloning machine and Copypasto spell do work with the Potion of Youth.
Essence of Life (base game)
The cowplant is basically the Sims version of Audrey II, the carnivorous plant from Little Shop of Horrors, and it's just as deadly--Sims who wander by it while it's hungry are likely to be eaten. When a cowplant murders a Sim, another Sim can obtain a potion called Essence of Life--which adds a few days to the Sim's lifespan each time they drink it. It would be tough for a Sim to live forever with this method, since it requires feeding the cowplant regularly, but it's possible.
Only scientist Sims can learn how to make the Age-Away Serum.
Age-Away serum (Get to Work expansion)
Like the Potion of Youth and Ambrosia, the Age-Away serum resets your Sim's current age back to zero. To obtain this recipe, you'll need your Sim to reach the maximum promotion in the Scientist career, and they'll eventually learn it by having a breakthrough at work.
The Age-Away serum requires alien ingredients that can only be found on Sixam, a planet that can only be accessed via rocket ship or using the Electroflux Wormhole Generator--both of which you will learn to build at work. You'll need Quill Fruit, which can be harvested on Sixam during the summer, any two alien crystals, and some xenopetrium. While the first two ingredients are easy to figure out, the xenopetrium is tougher.
Where to find Xenopetrium
This element is found in metals and crystals from Sixam, but you're extremely unlikely to be able to find it by analyzing those items yourself. Instead, send your space rocks to the Geo Council by clicking on them in your Sim's inventory, and they'll send back a pile of elements, almost certainly including xenopetrium.
The magic cauldron can help your Sim become an ancient, immortal wizard.
Potion of Rejuvenation (Realm of Magic)
Just like the Potion of Youth and the Age-Away serum, the Potion of Rejuvenation resets the timer on your Sim's current age. The ingredients for this potion are valerian root, alabaster, and lemon, and since it's a magical potion, it will have to be made in a cauldron by a spellcaster.
While alabaster and lemon are common and easy to obtain, valerian root is primarily found near the greenhouse in the Magic Realm--and you'll also get lots of random free ingredients from experimenting with the cauldron to learn new recipes, so there's a decent chance you'll get enough ingredients for one or two of these potions by the time you learn how to make it.
You can also buy all magical ingredients from the shop vendors in the Magic Realm.
Potion of Immortality (Realm of Magic)
This magic potion works differently from the ones we've previously discussed. It doesn't stop your Sim aging, but rather it makes them never die from old age--if you want to play as an ancient wizard, this is the path to take. It requires mandrake, sage, morcubus, and a diamond. Mandrake and sage are fairly common plants found all over, and morcubus and diamonds can be found by digging up rock piles anywhere in the world. And like the other potion ingredients, you can get all of these from the shop vendors in the Magic Realm
Vampires can both live forever
Become a vampire (Vampires)
While a vampire can die in an accident, they'll never age, meaning that simply being turned into a vampire will make your Sim immortal. Your Sim will still need to meet a vampire and convince them to turn them, but that's not too difficult. Vampires have one particular edge over the other forms of immortality, too: the ability to numb their emotions so they don't get upset for two days every time a member of the family tree dies.
Immortal Werewolf perk (Werewolves expansion)
Like vampires, werewolves can live forever--but it's not an inherent part of being a werewolf like it is with vampires. Instead, there's a perk at the end of the Werewolf skill tree, Immortal Werewolf, that will stop a werewolf from aging.
Orchid Arrangement (Seasons expansion)
Sims can use the Flower Arranging Table to, you guessed it, create flower arrangements, and excellent-quality arrangements can be scented for extra benefits. A Sim that has maxed out their flower-arranging skill can use a very rare orchid to scent an arrangement, and doing so will give the arrangement similar properties to the Potion of Youth--when your Sim gives it a whiff, they'll get a little bit younger, and may have their current age reset to zero. Orchids are pretty hard to find, but you can get one by grafting a lily onto a snapdragon plant (or vice versa). The resulting plant will occasionally produce orchids.
Those who have the Snowy Retreat expansion, however, will find a single orchid plant growing at the peak of Mt. Komorebi. Your Sim will need to level up their rock climbing skill to at least 5 or 6 in order to make it through the expedition, but this is the fastest way of obtaining a guaranteed orchid.
Mother Plant (StrangerVille)
The Mother Plant is the end boss in the StrangeVille expansion, but your Sim can become friends with it if they try hard enough. Sims who are friends with the Mother Plant can then ask it for a boon once each day--with one of the potential options being that the Mother Plant will extend their lives.
Source