Board Thread:General Discussion/@comment-170.10.254.61-20190611191519/@comment-4801337-20190611193901

You continue to grow up in your new host bodies. So, even though Kaylee was originally a child, she has lived other lives where she got to grow up and even get married.

I don't think the rule is that you have to be 21. You just need to be developed enough to take on the brain of the Prime. So, if the Prime was a child, another child brain would be sufficient as a host. Once you're an adult, that's when you need a fully developed brain (ages 18 - 21+).