Board Thread:General Discussion/@comment-27324545-20160105135410/@comment-44845046-20200119233142

I think Frisk is 10-12, Chara is 12 at death, Asriel is 11 or 12 at death, and Toriel is 200-9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999 years old.

Asgore is 1-3 years older than Toriel, Papyrus might be around 22 years old, Sans is in his late 20s or even in his early 30s, Undyne might be in her early or mid-20s, and Alphys might be around 35 years old.

Napstablook can be around his 30s, despite his UnderNet username being "NAPSTABLOOK22", Muffet might be in her 30s (very old for a spider), and Gaster is 6̶6̶6̶  around 40 or 50 years old, and Flowey might be around 12 or 13.