Board Thread:General Discussion/@comment-27213375-20160305063709/@comment-25181180-20160529095630

Muffet is likely still under 25. The creator said that Muffet 'might' be a child.

Monster Kid is likely the same age as us, probably 7 - 13. Chara acts more mature than their real age. Toriel and Asgore seemingly stopped aging when they're middle-aged, possibly 35 - 55. Several headcanons pointed that Sans views Toriel as his age, so he could be older than 25. Papyrus looks like he's a young adult.

Tsunderplane, judging from her personality, is probably still in high school (monster age). Etc.