Haven't heard of Matt Smith.
Good thought - they picked him despite his youth and emo hair because he's really talented, with masses of potential. Worrying contrary thought - they think young people won't watch a programme unless it stars a Young Person, and Matt Smith is the male Billy Piper.
I think making him young makes all relationships and romances much more problematic. It makes his interactions with older characters potentially - well, strange. Yes, I am thinking about The Master of course.
But perhaps Matt Smith can act as if he were both old and young. But again with the worrying contrary thought - perhaps it will become more squarely a kids' show.
No, surely not. I need to trust the process. Convince me you know best thou spirit of telly.