Seems a bit strange to be in love with a “kid” but honestly that is our own standards and culture judging it. For them, getting your Mark is pretty much being an “adult” and as far as nature is concerned, you’re an adult when you can do the deed and continue the species. That said, Karna is at the minimum age to be considered as such. Though his power is so great that he must be seen as a very desirable partner (Cecilia is still a demon who appreciates power even if she doesn’t just consider that), they have time to take it slow and gradually move things over years if need be. No immediate danger or need after all.