>>530407163
It's a good question, because while there are lots of clues towards them being children and the assumption feels obvious, on recollection I can't remember any hard, explicit evidence for this in the game.

So I'm checking the fan wiki now, and apparently a dev interview in The Escapist from 2013 states that all the fallen humans were indeed children.

https://www.escapistmagazine.com/undertale-dev-every-monster-should-feel-like-an-individual/