Young adults can be a parents for teens. It is weird.

I mean, it would be weird in real life, but not a game. So long as they are younger, it is fine. I would never want them to restrict it. If it feels odd, don't make them just one life stage apart. Also, especially for occult play, I would like parentage to be available on the same age group. Say what you will, but it makes perfect sense for a family of ageless, immortal vampires to all be YAs regardless of parentage.