I don't think Dragon Ball is a great comparison to shoot for due to it's drastically different internal logic, but I get the gist of what you're saying.
But the thing you're kind of overlooking is that ones character are often times defines and reinforced by who they make friends with and who they are romantically involved with. More dimensions of a character get explored in the specific contexts of who they're associating with, and how they associate with them. Jon's not gonna be quite the same person around Damian that he would be around his mom and dad. He's character opens up in a new and specific way around his friend that it doesn't around his parents. The same would be true for any romantic partners. And it would be different from Damian and his parents. People come with many faces, and those faces to fully show themselves at every situation.
But I get what you're saying insofar as him being 10 or 11 for you not too long ago, and I don't want it don't want it to be misinterpreted that I'm advocating that he jump into a relationship now, no, but the fact of the matter is that he's not 10 or 11 anymore. That's just the long and short of it. His character will establish and grow with the more situations he's put in, and the more character he has to bounce off of.