Considering how much time has passed in their universe, would it be safe to assume that MC and their sans are the oldest of the au bunch?
I'm not sure if this has been answered before, apologies if it has (-ā- ; )
i'm guessing you're asking about the playhouse fic? lol
but essentially? yes! MC and their Sans are the oldest AU variants, with the Horrortale brothers being second (having 10 more years over the others). i know their canon is 8 years after the neutral run, but this is deliberate on my part!
Third place would go to Undertale Sans and Paps because they already met their human, and said human had set them free! They're months ahead of the other members.
Underfell's and Swapfell's timelines are on the same milestones, which is to say, they're the exact age down to the seconds! They haven't met their human yet.
The youngest would be the Underswap brothers, as they're new to their neighborhood. Blue and his Alphys are just starting out as acquaintances, and Stretch recently distanced himself from the Lab.
If we're gonna put it in numbers...
MC and their Sans both died at their late twenties. If we're going to be technical about it, they already lived for 40+ years. But with how I see it, Sans died at his late twenties and MC just resumed his life from there. So at the start of the fic, Sans!MC are at their late twenties.
The ages are as follows (in the current timeline, from oldest to youngest):
Sans!MC: 98-102. Alphys died during MC's late eighties; thirteen years passed afterwards, so here we are.
Undertale: Late 20s (3 months after their human fell)
Underfell and Swapfell: Late 20s (6 months before their human fell)
Underswap: Late 20s (1 year before their human fell)