When it comes to maturity, I've noticed there's a certain level of professionalism and levity he tries to balance depending on the group.
If you put him with younger characters, like Miles or the New Warriors, he tends to fall into more of a mentor role and acts more serious, but if you have him around people his own age or older like the Avengers, he cuts loose with the gags. He's not always the same. It's a nice bit of characterization that I rarely see people mention.