Thread:Drdevilfx/@comment-913321-20170509103503/@comment-14577575-20170509205426

I think it's implied she is really supposed to be at middle school (in Darkness chapter 1, where Yui is uncertain about Momo being here). The anime doesn't show the age likely to avoid it getting a higher rating or being banned, and the video games are based on the anime and not the manga.