We've seen old play young countless times, how is this different? In fact, anyone playing the Doctor is young playing old, as a difference of 10 years doesn't really matter much in portraying a 900 year old. And even in our own understanding of the word Doctor, it is possible for a prodigious 26 year old to attain that title by that age if diligent enough. I can buy a 26 year old being called Doctor, is what I'm trying to say, if he seems special enough. All it comes down to is if Smith will be good in the role, and his age is not a factor in the quality of the performance, as far as we know.