In The Sims,only by using mods. In The Sims 2,type Ctrl+Shift+C and then type in the window boolprop testingcheatsenabled true.Shift+click on a sim,choose spawn and then Sim Modder.Choose Select Age and after that the age you want the sims to be.A mod that enables the genie wish Give me Youth does so too. In The Sims 3,use Ambrosia.Life Fruit and Deathfish are needed.
(Addenda from a different user)
In The Sims 3, this can be only done with either mods and cheats: To do it without a mod, you must have patched it at least at the Generation expansion pack level (i.e. Patch 21). Ambrosia cannot deage Sims (that is, from Elder to Young Adult), but it resets the age count to the beginning of the life cycle. If you have the patch, use the testingcheatsenabled and shift-click the Sim you wish to deage and go to the Create-a-Sim from there. If you have Generations expansion pack, you may also either purchase a Youth Potion for 70,000 happiness points OR make one from the Chemistry lab: to make it the first time, a Sim must have a logic level of 10 and first discovery will be random: it might end in an explosion. If you have a Sim who already knows the method but doesn't have the potion, it costs 5000 simoleons, or 3750 if he/she completed the Potion challenge. I have not used the Genie method yet.(user talk:Mathetesalexandrou) 04:27, March 13, 2012 (UTC)
In The Sims 3 Pets you can age down your pets with the Young Again Pet Potion, found by exploring places like the lighthouse or the beach cave, but not your sims.