If you don't want your sim to age at all, you could turn aging off in the options menu.
If you own the Generations Expansion Pack and want your sim to remain young and beautiful forever, but have the rest of the neighbourhood wither and die, you can craft a
Young Again Potion on the chemistry set which resets your sims "green bar" to the first day of the Young Adult Life Stage when you drink it, another option to avoid all that is have your sim save up 65000 Lifetime Happiness Points and buy the Age Freeze Potion, which stops them from aging
FOREVER.
And my final idea to (hopefully) help your issue, is to have your sim max the cooking, gardening, and fishing skills, buy the
Ambrosia recipe book at the Book Store, and make a meal.