Board Thread:General Discussion/@comment-27324545-20160105135410/@comment-27524978-20160529030641

No ages are really confirmed, and Frisk and Chara's are up to interpretation.

But either way, here's what I think their ages are:

Frisk, Chara and Asriel: Around 8-12

Undyne, Alphys, Papyrus and Sans: In their 20s or so

Asgore and Toriel: In their 60s-100s (they look young since they are boss monsters after all)

Flowey: Immortal, probably older than Asriel.