Legacy Challenge

Do you have the vampire pack? Vampire sims are immortal so you could play a legacy without anyone ever aging beyond a YA

This would be some work, but turn off aging, keep track of how many Sim days go by, and manually age them up each time until they reach adult. Once they hit the number of Sim days when they should become an elder, you could maybe turn their hair grey and give them some wrinkles so they superficially look older.

If you're trying to play by the challenge rules to some extent, give only a certain amount of time to each heir before moving on to the next one.

You have them live forever if you want. Legacy is playing the same family bloodline over the course of several(10) generations. You can make your own rules to it if you want.

I only play legacies, and I play them by my own rules. Basically, if the game allows it without cheats, it's fair game. I also let my Sims have as many kids as I think they should, but I play in rotation every branch of the tree. I don't do the heir and the spare thing. I've never played with aging off though. I hate when my Sims die, but dying helps continue the stories I make up.