Board Thread:General Discussion/@comment-27213375-20160305063709/@comment-26347196-20160417184057

To be honest, if I were to stick to the canon as much as I can, you can see that in the UnderNet usernames, Papyrus bears COOLSKELETON95, Undyne bears Strongfish91 and Napstablook does have one (which I don't know) (And Alphys does not have one as well). Given one of the common method of naming usernames on certain social medias, I think I can guess ages that are relatively canonical.

Papyrus: 21

Undyne: 25

Napstablook: 94