There are now two actors who played the Doctor who can appear in future installments of 'Doctor Who' at any time in their careers, without having to worry about any sort of wibbly-wobbly, timey-wimey splainins as to why they looked older (as was needed in the past.)
Those two former Doctors would be Paul McGann and David Tennant.
With Paul McGann, he only had the one shot at the role onscreen - the 1996 TV movie. (He's since done many Big Finish audio adventures.) We saw him emerge as the result of the Seventh Doctor's regeneration, but he remained looking like this through the end of the movie:
And he would be the Doctor through the next nine years (albeit off-screen.) When 'Doctor Who' returned in 2005, the Doctor was already regnerated by the time he re-appeared on our TV screens, so we never saw what Paul McGann looked like at the end of his tenure as the Doctor.
So McGann could get white-haired and have his face become deeply lined, and it wouldn't matter whenever - if ever! - he returned to the role.
As for Tennant, the Grand Moff and the Powers That Be just have to bring him back as the human Doctor clone from "Journey's End", now living with Rose in her new home dimension. That version of the Doctor is going to age and have only a normal life-span for a human. As Tennant ages (He's 40 now!), so too will this second Doctor (not to be confused with the Second Doctor!)