Doctor Who: 10 Things You Didn't Know About The Twelfth Doctor
5. He's The Joint Oldest Doctor
There's a gag in Twice Upon A Time in which the First Doctor eyes up his future incarnation and muses "Well, I assumed I'd get younger", to which the Twelfth Doctor replies "I am younger!"
It's a joke about how, when Peter Capaldi first appeared as the Twelfth Doctor in The Day of the Doctor, he was only three months younger than William Hartnell had been when he played the First Doctor back in 1963.
Both Capaldi and Hartnell were 55 at the time they first assumed the role, with Hartnell being born on 8 January 1908, and Capaldi being born on 14 April 1958. Their similar ages brilliantly puncture the myth that William Hartnell was some ancient old man when he was cast in the role – though giving him a long white wig didn't really aid this perception!
Case in point, David Bradley was 75 when he appeared as the First Doctor in Twice Upon A Time, adding an extra dimension to the joke about who was actually "younger".
In addition, both Jon Pertwee and David Tennant were only a few years younger than Capaldi and Hartnell when they played the Third and Fourteenth Doctors respectively.
And yes, realising that David Tennant is now the same age as the First Doctor really is quite unnerving.