Board Thread:Questions and Answers/@comment-27605058-20160221175024/@comment-25942629-20160221210515

Perhaps. We don't know enough about vampires to say. Apparently, they revert to the age when they were most pure and most full of desires, so he might actually grow younger. That's why most of the demons we've seen thus far (aside from Mahiru-no-yo) appear to be around 12 years old. Their androgyny is related to some kind of belief about demons as well.

Even though Guren is a naminari, he has continued to age, so I wouldn't put Yu's growth on hold just yet. We don't really know enough to say.