Legacy Challenge

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.