He's 59 now, they've never cast anyone that old
He would have made a good Doctor though, also a good Master!
It's too bad age is a factor when it comes to casting. Patrick Stewart was in his late forties and early fifties during TNG. So I don't know why age (within reason) would be a big deal. So as long as the actor isn't being followed by someone with a dirt and shovel then don't worry about age.
Age is unfortunately a factor because of the working hours. Doctor Who
today is much a faster-paced, stressful production than The Next Generation
TNG put out one episode every 8 days, 26 episodes (on average) a season (series for you UKed challanged). Doctor Who puts out (if we're lucky) 14 episodes every 18 months.
The only reason a Doctor can't be a little "older" is because of one of the Doctors main functions: RUN!