Excuse me? Are you calling Sirius mature? Are you?
Well, sure, he's sinister, but that doesn't necessarily mean the same thing as mature. Of course, I still get a rush out of seeing a thirty-six-year-old man who's been in prison for twelve years, essentially the cause of his best friends' deaths, with a face like a death mask . . . worrying like a mother hen over his godson. (**Squeal, pinches Sirius' cheeks**)
And as for the schoolboy grudge, he's even worse than Snape. Nah, my theory is that as far as life experience goes, his stopped dead between 1981 and 1993, and resumed as soon as he got out. So nowadays he's still capable of acting like a frat boy. Do you hear me, Sirius? FRAT BOY!
So . . . no. I wouldn't say he's mature at all. Just you wait until Remus comes back into the scene; he'll be nannying Sirius all over the place.
"For god's sake, clean your room!"
"Don't wanna!"
"Clean your room or you don't get any Death Eaters to beat up!"