Board Thread:General Discussion/@comment-27324545-20160105135410/@comment-13682699-20180430094959

AstraStars wrote: Here's what I think and my logic behind it

Frisk: 14-16 because if they were any younger I don't think they would be able to carefully craft a plan to either WIPE OUT AN ENTIRE RACE or save all the monsters

Sans: 18-22 because it's implyed that he did work at a lab once, but not for too long, another reason is that he's SUPER lazy(heh, like me) and I don't  think a middle aged skeleton would be like that but if he was a teen he wouldn't be a sentry

Papyrus: 16-18 just a bit younger than Sans.... I have no logic, just a guess

Toriel: 45 physically. 1000 in reality, we know that she was around to see the war so that happened I'd say AT LEAST 850 years before undertale

Asgore: 60 physically. 1050 in reality

Chara(before death): 12

Chara:(presently): ageless but looks 13

Asriel: same as Chara

Flowey: Immortal

Undyne: I'd say probably 21-25

Alphys: 22-24?

W.D. Gaster: 666 or 6666

Mettaton: Robots have no age but I'd say he's programmed to act 21

Muffet: 25-30?

I don't know why you would think Asgore would be physically SO much older than Toriel when they both had and lost their children at the same time. Boss monsters don't age after a certain point until they have children, which means they are both physically around the same age.