Normally, when the goal of the challenge I'm playing has been completed.
However, I put all my completed challenge Sims in my custom templates to mostly act as NPCs not-in-world, but sometimes they are housed. I use these templates to start new saves. Sometimes their story will continue with the new challenge household if they become a friend and/or spouse of the current challenge Sim. I don't play just one save or rotationally, so my gameplay has many beginnings/endings and/or continuations.
Aging on or off isn't a set rule for my gameplay, it depends on the rules of the challenge. For instance, last year and again this year with my own challenge, there is a timeline when the challenge Sim ages up and I'll accomplish that in CAS or with a cake, depending on the story I'm writing for the character. Last year I couldn't bear to have my challenge Sims die of old age as intended, so I wrote into the story they would become vampires again and live forever. I don't know if I'll do something similar with this year's challenge Sim yet, it depends on where the story goes as I write for him.
If I'm just playing a casual save, aging is off until I decide if they will age or not depending on the goals I've set for them. There is always a goal when I play, be it from challenge rules or my own ideas/story for their little pixel lives. :)
Having said all that, sometimes I end up just not liking the challenge I'm playing and quit completely to try another one or a new pack comes out and I want to try a challenge using it or just check out the gameplay. In that case, I'll put the current challenge I'm playing on hiatus, or just play more than one save at a time. The last two years it's been the latter, or I'll replace one world in the timeline of my own current challenge with the new world from a pack (if it has one) so my challenge Sim can live there and do the aspiration(s)/gameplay for it. That's what I intend to do with the Werewolves pack.
I will also start a brand new save in Moonwood Mills for a story I've wanted to write for a long time. I couldn't write it though without Shapeshifters, which I believe can be accomplished using assets from the Werewolves pack now. We shall see... :)
