Some random thoughts here about the Doctor’s ages. Not the character, but the actors who played him / her.
Updated to include the 14th Doctor (as best I can at this point).
(Daily Doctor Who #233)
I found myself wondering how old each actor was when they filmed their first scene as the character, and did some quick research to find out. Here they are, from oldest to youngest, as best as I could determine.
William Hartnell – the First Doctor
Birthday: January 8, 1908.
Filming Debut: October 9, 1963 (This refers to the second time the original episode, An Unearthly Child, was taped, making it his debut in the role as far as what was eventually seen on TV, not as a DVD extra).
Age at filming: 55 years, 9 months, 1 day.
Peter Capaldi – the Twelfth Doctor
Birthday: April 14, 1958
Filming Debut: October 3, 2013 (both his cameo in Day of the Doctor, and his regeneration scene in Time of the Doctor)
Age at filming: 55 years, 5 months, 19 days.
Jon Pertwee – the Third Doctor
Birthday: July 7, 1919.
Filming Debut: September 13, 1969 (Spearhead from Space)
Age at filming: 50 years, 2 months, 6 days.
Sylvester McCoy – the Seventh Doctor
Birthday: August 20, 1943.
Filming Debut: April 4, 1987 (Time and the Rani)
Age at filming: 43 years, 7 months, 15 days.
Patrick Troughton – the Second Doctor
Birthday: March 25, 1920
Filming Debut: October 9, 1963 (Power of the Daleks)
Age at filming: 43 years, 6 months, 15 days.
Colin Baker – the Sixth Doctor
Birthday: June 8, 1943.
Filming Debut: January 12, 1984 (The end of The Caves of Androzani)
Age at filming: 40 years, 7 months, 4 days.
Christopher Eccleston – the Ninth Doctor
Birthday: February 16, 1964
Filming Debut: July 20, 2004 (Rose)
Age at filming: 40 years, 5 months, 4 days.
Tom Baker – the Fourth Doctor
Birthday: January 20, 1934.
Filming Debut: April 28, 1974 (Robot)
Age at filming: 40 years, 3 months, 8 days.
Paul McGann – the Eighth Doctor
Birthday: November 15, 1959
Filming Debut: January 15, 1996 (The TV Movie)
Age at filming: 36 years, 2 months.
Jodie Whittaker – the Thirteenth Doctor
Birthday: June 17, 1982.
Filming Debut: July 19, 2017 (The end of Twice Upon a Time)
Age at filming: 35 years, 1 month, 2 days.
David Tennant – the Tenth Doctor
Birthday: April 18, 1971.
Filming Debut: April 21, 2005 (The end of The Parting of the Ways)
Age at filming: 34 years, 3 days.
Ncuti Gatwa – the Fourteenth Doctor
Birthday: October 15, 1992.
Filming Debut: No idea yet.
Age at filming: 29 years, some number of months and days (presumably)
Peter Davison – the Fifth Doctor
Birthday: April 13, 1951.
Filming Debut: April 13, 1980 (Four to Doomsday–his second story, but the first one filmed)
Age at filming: 29 years exactly!
Matt Smith – the Eleventh Doctor
Birthday: October 28, 1982.
Filming Debut: May 12, 2009 (The end of The End of Time Part 2)
Age at filming: 26 years, 6 months, 15 days.
I haven’t bothered looking up everything to do with some of the other actors to play the Doctor. I’ve read that John Hurt was the oldest to play the role, being around 73 when he appeared.
But as best I can determine, David Bradley (the First Doctor in Twice Upon a Time) would have been nearly 75 when he debuted.
Richard Hurndall (the First Doctor in The Five Doctors) would have been 72.
And Jo Martin, who played the fugitive Doctor in Fugitive of the Judoon, doesn’t seem to have her birthday publicly listed.
However, she appears to be an adult in the episode of The Brittas Empire that she appeared in 1990, so I’m guessing she is in her early 50’s now, making her maybe a little older than Jon Pertwee was when he took on the part.