It seems an interesting role to full, as there are no limitations on the actors appearance, age, etc (even gender if you believed the rumours) as he could regenerate in to just about anything. His age shouldn't therefore matter as long as he can adopt a convincing persona on screen (and even that changes with each regeneration).
Going back to the rumours of a female doctor, I picked up a copy of the DVD of Caves of Androzani in a second hand shop a few months ago. Included in the extras was a TV interview with Peter Davidson and then producer John Nathan-Turner on one of the BBC local new programmes from shortly after Davidson announced he was leaving the show. One of the things mentioned in the interview is the fact that some of the papers had carried stories that the next doctor was going to be a woman, so just goes to show nothing changes in 25 years.
Davidson did also mention that the relationship with the companions would be awkward if the doctor was much younger than him, but then that relationship seems to have changed quite a lot since then anyway.