In Sims 2 you could make Elixir of Life to stop Sims from aging. Is there any way to do this in Sims 3?
Making a Sim "Younger"
You can reduce a Sim's age by a single day by eating a raw Life Fruit. However, they can only be planted by a level 7 Gardener and the "Unknown Special Seeds" or "Life Seeds" are only available under certain circumstances:
Sims can receive the seed by searching the town, as a science career reward, by exploring the catacombs in a mausoleum, fishing it out of a body of water, or while using the Time Machine.
Source: Sims 3 Wikia, Life Fruit page
The better option is to cook Ambrosia, which resets the Sims age to a predefined value (adults eating it will reset to have 26 days until aging). It can also bring Ghost Sims back to life.
Ambrosia requires purchasing the recipe (§12000), a level 10 Cooking Skill and then one Life Fruit and one Deathfish as ingredients.
A deathfish can only be caught with level 10 Fishing skill, from the Graveyard between midnight and 5AM and needs angelfish bait. Note that if you catch enough deathfish you can stock a pond and have a supply of these available all day round.
Young Again potion
This potion requires the Generations pack and is created using a chemistry set, it resets a Sim who is older than a young adult back to day 1 of the young adult.
You will need a high Logic skill for this.
Prevent / slow down further aging
Age Freeze Potion
Another potion, so also requires the Generations pack. This is a lifetime reward that requires 65000 lifetime happiness to obtain. Drinking it will completely stop your Sim from aging further. Note that if you eat birthday cake after drinking the potion, your aging will start up again.
If you have the relevant expansion pack, you could also get the Sim converted to a Vampire or Mummy, as they have longer life spans.
If you enable the "god mode" cheat (testingcheatsenabled true) in the cheat console, which is activated by pressing control, shift and c in unison, you can decrease the sim's age, but only in the given stage of life. By shift clicking the sim, and editing them (you don't even have to change anything) in create a sim, the game will set their life back to the first day of whatever stage they are in.
Sort of. There is the Ambrosia, which you can make yourself, although it is a little tricky.
Here is a quote from another helpful player
There is the Ambrosia recipe that you can buy from the book store. It's a level 10 cooking recipe and when you make it it resets your current life meter. So if you're 20 days into the Adult stage of life, it sets it back to 0. Also, if you have a sim ghost eat it, they will be brought back to life.
Of course, to be able to make it requires some items that are not easily come by. Namely you need a life fruit which can only be grown from a special seed found somewhere on the ground and grown by a high level gardener, and you need a Deathfish, which I believe can only be found in the graveyard pond after midnight and probably only fished by a higher level fisherman.
I hope that helps.
Assuming you have the generations expansion, there are two aspiration rewards: The Anti Aging Potion and the Young Again Potion.
They are both pretty pricey, but less so than Ambrosia, which is, comparatively, underpowered and overpriced.
On closer reflection, these two might actually be available in the base game patched up to full, but I'm not certain on this point.
You can open the cheat bar and put:
Testingcheatsenabled True. and go to the Life Happiness and right or left click between the chest and amount until you get enough to get the like last one, You'll see it or..You can also enter the same thing but put Motherload and keep doing that untill you get 45,000 simolins or whatever and purchase that Fountian Of Youth if you have the Generations Expantion pack (i think you may have to buy that or not...)