An interesting but useless fact I learned whilst trawling Wikipedia.
Peter Davison, who played the fifth Doctor in Doctor Who, was and is the youngest to play the role, and when he reprised it in the mini-episode "Time Crash" (alongside David Tennant) he was older (at 56) than William Hartnell (at 55) when he first played the role of the first Doctor in "The Unearthly Child" serial.