When Do You Decide To Age Your Sims to the Elder Lifestage?

I play with aging on and a normal lifespan. Teenagers usually get one or two potions of youth so that by the time they are young adults and ready to start families of their own, their parents are ready to kick the bucket. (This is to keep the size of the family down; after about four or five Sims, I have a really hard time taking care of all of them!) But I do let the adults live out their natural lives, with birthday cakes being restricted to the day that they would age-up anyway.