Yes, it's weird, and Adults can be parents to Young adults, too. It's all about not being on same lifestage.
I think it's fine though. In my game sims turn teen at 15, and YA at 29. I think it's default by the game, not sure why they thought sims at 28 should be teenagers... My young adults are between 29 and 50. So, at max, the teen at 15 can hava a parent at 50. I don't see a problem with that. Anyway, it's a sand box, even if the game allow for many things, it doesn't mean I have to do it. In my game I never create fresh young adults having teens.
