I'm just gonna add another person saying to not take it seriously. I really hate people whining about these kinds of things. I view it as purely fictional, I don't question any silly real-life society restrictions in a fictional scenario in a fictional world with fictional people in it, I just marry who I like as a couple. my favorite husband for Nowi is Lon'qu, and that's probably one of her most childish supports. I just found it adorable. Who cares what other people think?
Besides, look from their perspective. Nowi and Nyx are stuck in these bodies forever. So they should NEVER get to marry anyone because they look young? That's silly thinking. Especially for Nyx, who appears to have matured mentally. She might find it gross to date little boys similarly to how people think it would be gross to marry her. They're not actually kids so I don't view them as kids to begin with. Also, I'm a baby-faced individual myself at 21, and I certainly hate being viewed as a little kid, not being able to get married because of my baby-face would be devastating.