Did I mention I loved undertale, at any point and time?
More Ageretale lore (kinda, more like facts?) :3
(Undertale) Sans regresses because of loosing his father due to the incident at the core, having grow up faster in order to take of Papyrus at a young age, becoming hyper aware of the world they’re living and existential dread, Frisk’s (Geno) run, and the often resets.
In Undertale, before they’re let out onto the surface; Toriel found out Sans was an age regressor before sans was even able to tell.
In moments of impure regression, which is how Toriel found out about Sans regression, (He had a huge breakdown near the ruins) She comforted him best he could and sang to him until he calmed down, then afterwards read a book of jokes and puns to him.
When they make it to the surface Frisk finds out about Sans’s regression (Papyrus accidentally told her) and She’s also cool with it, now she can hang out with her brother figure who’s interested in doing things kids her age do without him ever feeling too grown up to play with her. She also understands that this is just how Sans copes with some of the issues she’s caused, which resulted in her making a promise to never reset again.
Underswap a way different though. Sans age dreams for fun,and especially on bad days. Papyrus doesn’t mind it because he supports Sans in literally, and even admires him for being able to let out his inner child.