When William Hartnell passed away in 1975 at the age of 23rd April 1975 at the age of 67, Patrick Troughton was 55 (coincidentally the age Hartnell was when he took on the role of the Doctor), Jon Pertwee was 56, Tom Baker was 41, Peter Davison was 23-24, Colin Baker and Sylvester McCoy were both 31-32, Paul McGann was 16, Christopher Eccleston was 11, David Tennant was 4, and Matt Smith was not born until 1982, making him the only Doctor not to have shared this planet with the originator of the role.
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Patrick Troughton was 55, Jon Pertwee 55, Tom Baker 41. As for the future Drs, Peter Davison was 23(almost 24), Colin Baker and Sylvester McCoy both still 31.
Paul McGann was 23, Christopher Eccleston was 11, David Tennant was 4 and Matt Smith hadn't been born (and wouldn't be for another 7 years).