Actually, he's probably a bit older than that (he was about 12 by the end of Johto, I think). He's probably somewhere around fifteen. Also, I wouldn't say he's getting more immature. Sure, he's dense, but he certainly isn't growing more child-like. He is maturing, I think. Dawn is ten, the first episode (in the English dub, at least) of D/P says so. And, Ash is much too focused on battling and becoming a Pokemon Master to think about personal relationships that would probably just complicate things.
...But that's another conversation.
I personally will lose interest if they get rid of the current characters completely. I can live without Ash (so long as he shows up every so often), but if the story were to focus on Max's journey (with the Ralts and everything?) with Ash occasionally stopping by, that's be fine. So, I wouldn't mind the focus being taken off Ash at all, but I don't mind if it remains on him.