How old was Colin Baker when he played the Doctor?
It’s a slightly trickier question to answer than it looks at first glance. Do you mean when he was cast? When shooting began? His cameo in Androzani, or his first proper episode? Or maybe when his first episode was actually broadcast?
There isn’t much difference, admittedly. The answer is 40 (if you’re being sensible and don’t want months or weeks or days), but if you’re making an ordered list of how old the various actors were when they first played the Doctor, it does present a tiny problem. As depending on when you count, he’s older/younger than when Eccleston played the role (also 40, but 41 when his first episode was broadcast).
Happily, the point of this particular list is less about worrying precisely when we should consider a Doctor’s tenure to have begun, and more about how I find it funny that David Tennant is now the third oldest Doctor. Third! Lols! …well, it amuses me anyway. Amuses, and fills me with existential dread about the passage of time. Hurrah!
Given that I’ve met Doctor Who fans, I want to put a long, long disclaimer at the start of the post: the information presented here is the result of a combo of Internet research (and there is plenty of reliable info on the Internet about Who, shush) and a quick flick though those lovely Howe-Stammers-Walker Handbooks from the nineties. All mistakes are mine and there are almost certainly mistakes. These mistakes may include, but are not limited to, counting the Doctor’s first proper episode rather than an end of episode cameo; and counting the first day of shooting an episode rather than the first day the actor was actually recording.
The important thing though – the very important thing – is that David Tennant is now the third oldest actor to have played the Doctor*. Remember that for your Doctor Who trivia quizzes. Don’t let anyone steal your point!
*Oh God, by which I mean actor to have played the Doctor as a main role and not a guest or supporting part**. I both love and hate this fandom for this sort of ridiculousness.
**But there are Bonus Doctors at the end anyway.
The Quick List (age at casting/shooting/broadcast date):
William Hartnell – 55/55/55
Peter Capaldi – 55/55/56
David Tennant – 51/51/52
Jon Pertwee – 49/50/50
Patrick Troughton – 46/46/46
Sylvester McCoy – 43/43/44
Colin Baker – 40/40/40
Christopher Eccleston – 40/40/41
Tom Baker – 40/40/40
Paul McGann – 36/36/36
Jodie Whittaker – 35/35/36
David Tennant – 33/34/34
Ncuti Gatwa – 29/30/31
Peter Davison – 29/30/30
Matt Smith – 26/26/27
Way too many details:

William Hartnell – First Doctor (55)
Birth date: 8th January 1908
Casting announced: 21st November 1963 (press conference to launch the show.)
Shooting begins: 27th September 1963
First episode broadcast: 23rd November 1963
Age at casting announcement: 55
Age when shooting begins: 55
Age at first broadcast: 55

Peter Capaldi – Twelfth Doctor
Birth date: 14th April 1958
Casting announced: 4th August 2013
Shooting begins: 7th January 2014
First episode broadcast: 23rd August 2014
Age at casting announcement: 55
Age when shooting begins: 55
Age at first broadcast: 56
David Tennant – Fourteenth Doctor
Birth date: 18th April 1971
Casting announced: 23 October 2022
Shooting begins: June 2022 (?)
First episode broadcast: November 2023
Age at casting announcement: 51
Age when shooting begins: 51
Age at first broadcast: 52
Jon Pertwee – Third Doctor
Birth date: 7th July 1919
Casting announced: 17th June 1969
Shooting begins: September 1969
First episode broadcast: 3rd January 1970
Age at casting announcement: 49
Age when shooting begins: 50
Age at first broadcast: 50
Patrick Troughton – Second Doctor
Birth date: 25th March 1920
Casting announced: 2nd September 1966
Shooting begins: 22nd October 1966
First episode broadcast: 5th November 1966
Age at casting announcement: 46
Age when shooting begins: 46
Age at first broadcast: 46
Sylvester McCoy – Seventh Doctor
Birth date: 20th August 1943
Casting announced: 27th February 1987
Shooting begins: 4th April 1987
First episode broadcast: 7th September 1987
Age at casting announcement: 43
Age when shooting begins: 43
Age at first broadcast: 44
Colin Baker – Sixth Doctor
Birth date: 8th June 1943
Casting announced:19th August 1983
Shooting begins: 24th January 1984
First episode broadcast: 22nd March 1984
Age at casting announcement: 40 (2 months, 11 days)
Age when shooting begins: 40
Age at first broadcast: 40
Christopher Eccleston – Ninth Doctor
Birth date: 16th February 1964
Casting announced: 2nd April 2004
Shooting begins: 20th July 2004
First episode broadcast: 26th March 2005
Age at casting announcement: 40 (1 month, 17 days)
Age when shooting begins: 40
Age at first broadcast: 41
Tom Baker – Fourth Doctor
Birth date: 20th January 1934
Casting announced: 15th February 1974
Shooting begins: 1st June 1974
First episode broadcast: 28th December 1974
Age at casting announcement: 40 (26 days)
Age when shooting begins: 40
Age at first broadcast: 40
Paul McGann – Eighth Doctor
Birth date: 14th November 1959
Casting announced: 5th January 1996
Shooting begins: 15th January 1996
First episode broadcast: 12th May 1996
Age at casting announcement: 36
Age when shooting begins: 36
Age at first broadcast: 36
Jodie Whittaker – Thirteenth Doctor
Birth date: 17th June 1982
Casting announced: 16th July 2017
Shooting begins: 31st October 2017
First episode broadcast: 7th October 2018
Age at casting announcement: 35
Age when shooting begins: 35
Age at first broadcast: 36
David Tennant – Tenth Doctor
Birth date: 18th April 1971
Casting announced: 16th April 2005
Shooting begins: July 2005
First episode broadcast: 25th December 2005
Age at casting announcement: 33
Age when shooting begins: 34
Age at first broadcast: 34
Ncuti Gatwa – Fifteenth Doctor
Birth date: 15th October 1992
Casting announced: 8th May 2022
Shooting begins: November 2022
First episode broadcast: 2023 (?)
Age at casting announcement: 29
Age when shooting begins: 30
Age at first broadcast: 31 (?)
Peter Davison – Fifth Doctor
Birth date: 13th April 1951
Casting announced: 4th November 1980
Shooting begins: 13th April 1981
First episode broadcast: 4th January 1982
Age at casting announcement: 29
Age when shooting begins: 30
Age at first broadcast: 30
Matt Smith – Eleventh Doctor
Birth date: 28th October 1982
Casting announced: 3rd January 2009
Shooting begins: 20th July 2009
First episode broadcast: 3rd April 2010
Age at casting announcement: 26
Age when shooting begins: 26
Age at first broadcast: 27
Bonus Doctors!
David Bradley
Birth date: 17th April 1942
First episode broadcast: 1st July 2017
Age at first broadcast: 75
John Hurt
Birth date: 22nd January 1940
First episode broadcast: 18th May 2013
Age at first broadcast: 73
Richard Hurndall
Birth date: 3rd November 1910
First episode broadcast: 23rd November 1983
Age at first broadcast: 73
Peter Cushing
Birth date: 26th May 1913
First film released: 23rd August 1965
Age at first broadcast: 51
Jo Martin
Birth date: 29th April 1980 (? – online ref but possibly unreliable)
First episode broadcast: 26th January 2020
Age at first broadcast: 39 (?)
I always find it mind-boggling that William Hartnell was only 55 when he was cast as the Doctor. He looked so much older. Peter Davison is now 71 years old and he *still* looks younger than William Hartnell!