Board Thread:General Discussion/@comment-27324545-20160105135410/@comment-26244866-20170626225328

Frisk: 8-9 (and gender-neutral)

Chara: Idk but I see her (I see Chara as a girl) as slightly older than Frisk (physical age)

Asriel: 7-8

Monster Kid: Around the same age as Frisk (and male)

Toriel: 1000+ chronologically, physically around 26 (as boss monsters age until adulthood, then stop aging and then start aging again when they have a child as long as the child is aging)

Asgore: Same as Toriel

Sans: Approximately 25, a few years older than Papyrus

Papyrus: 20

Undyne: 24

Alphys: Unknown, but I thought she looked much older than Undyne in the game for some reason... Well, anyway, I think she's probably around the same age as Undyne and Sans or slightly older - Bratty and Catty are also around the same age as Alphys (but Bratty and Catty have to be young enough for 19-year-old Burgerpants to have a sort-of crush on them)

Muffet: Probably somewhere around 18, as she is too old for the striped shirts (which is a thing for monsters) but it was said that she might be a child

Bugerpants: 19 (no matter what people say, this is the only official canon age - or maybe there's a less notable character with a canon age that I've been missing.)

Gerson: He is extremely old, as he was there during the war between humans and monsters and unlike boss monsters, he is not ageless (well, boss monsters aren't really ageless but I mean like he doesn't stop aging at any point).