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.
That's misleading though. Consider how many scenes each actor is in. TNG was an ensemble show. Yes, Picard might get more episodes focused on him, but, it got balanced out with others. And, there was always a B plot involving others.
In Doctor Who, the Doctor is many, many of the scenes. That's a lot of time in front of a camera. That's a lot of work.