Board Thread:General Discussion/@comment-27324545-20160105135410/@comment-35858934-20180710054442

Undyne: 21-26

Alphys: 25ish

Asgore: 40

Toriel: 38

Sans: 26-28

Papyrus: 20-23

Temmie: 15

Frisk: 13

Monster Kid: 13

Chara (when they died): 10

Asriel: 11

Flowey (years of being created): 3

Mettaton: 19

Napstablook: 20

Catty and Bratty: Mid twentys

Muffet: 17

This is just my honest interpritation of things. I have no actual clue. I just always saw things like this, and the characters in this age range.