There's video with Smith talking about the selection
here. He doesn't come across as adolescent or juvenile to me, and I love the way he lights up just mentioning the joy at becoming the new Doctor and not being able to tell anyone.
Maybe I'm taking this all in stride because I was actually following Doctor Who when Peter Davison took the role, and I remember the way that fandom (though in limited magazine/letters form at that time) castigated him at the time for replacing Tom Baker. I always liked him and felt sorry for the fact that people were dead set against him -- both because of his age and he was considered too conventionally "handsome" to be the Doctor.
I never saw Russell's move as "social change," necessarily, but rather as a welcome moving of the doctor into a "real" setting, more or less. It wasn't just fake aliens and bad sets. (Though the fight with the dummy hand in the first episode certainly harkened to those old days.) Yes, deaths occurred in those earlier incarnations, but they never had the real sense of loss -- and its toll on The Doctor -- during those days. It was very much a faked setting, and not the real "world." With the new Who, there were real issues that were confronted, and by allowing the companions to maintain contact, we could also see the cost on those who traveled with him.