There's one other aspect that everyone (including me in my prior post) seems to have forgotten, and that's one of logistics.
Donna Noble set out the Doctor's job description in "The Doctor's Daughter": "There's an awful lot of running involved".
This isn't an innovation that came in with RTD. Go back all the way to Hartnell and there was, indeed, a lot of running involved. Remember Troughton trying to outrun the Cybermen's laser guns on the streets on London?
The fact is the Doctor cannot allow himself to regenerate into someone who isn't physically capable of keeping up with the lifestyle he has set out for himself. Remember in 11th Hour after the duck-pond exchange where the Doctor sets off at a run and hops a fence at full gallop? I can't even see Eccleston doing that. Let alone someone like Richard Griffiths (Vernon Dursley in the Potter films) or Bill Nighy, both of whom were mentioned as possible Doctors at one time. Could you imagine Troughton or Pertwee or Hartnell recovering after falling through multiple rooms in the TARDIS and landing in the swimming pool, and then climbing out using block and tackle? Possibly Pertwee, but not the others.
In the novels (I'd put a spoiler tag but the books in question are a decade old now) Romana II (the cute blonde with the mischievous grin) purposely regenerated into a harder-edged Romana III because she knew II didn't have the personality to handle the Time War that was coming up. Whether subconsciously or intentionally, the Doctor clearly is trying to skew either younger or about the same age so that he might be able to handle the rigors of being the last Time Lord.
Alex