how i can make my sim younger ?

[Minty]

Do you mean a Sim you are creating, or in game?

If it is a Sim you are creating, go to CAS (Create-A-Sim), load up the saved version of the Sim you wish to change, and then simply change from from elderly or whatever down to Adult, Young Adult, Teen, Child or Toddler. Remember that if they go below Young Adult they will lose traits.

If it is in game, if you have the Generations expansion pack, you can go for the "Young Again" potion with the chemistry set, and the Hidden Springs world map has the Fountain Of Youth where you can wish for youth & restart the lifestage you are one back to day one in terms of aging.