You can use the chemistry table from Generations. If your sims has mastered the logic skill, they can create a young again potion.
You can also use your lifetime points to buy one. There's an age freeze potion that will stop your sim from aging.
Ambrosia reset a sim's age. If they're elders, they go back to the beginning of the bar. If they're young adults, they also have their age bars resetted. An elder doesn't become a young adult with it.
I believe there are other ways to become younger or reset your sim's current age. There's a mud bath that reduces a day, I think it's from Hidden Springs too. Your sim can have a diet of life fruit. You can also use Master Controller and cheat.