But it doesn't fully understand young and "naked young person" isn't just a scaled down "naked adult".
Do you actually know that, or are you just assuming it?
Personally, I'm basing my assertions off of experience with related situations, where I've asked image AIs to generate images of things that I'm quite sure weren't in its training set and that require conceptual understanding to create "hybrids." It's done a decent job of those so I'm assuming that it can figure out this specific situation as well, since most of these models have a lot of examples of naked people and young people in their training sets. But I haven't actually asked any AIs to generate images of naked young people to test this one specific case.