Xavier wasn't "an old guy", he just looks old because he lost his hair, which is caused by his mutant powers and not age.

The subject just came up recently on Usenet and I did this calculation:

The OHOTMU page
http://marvel.com/universe/Professor_X says he entered college at age 16 and
graduated in two years at 18.

At this point we no longer have exact numbers. He went to Oxford and met
Moira MacTaggert. Let's say two years (he didn't finish).

Assume one more year for the military. One more for meeting Gabrielle Haller
and Magneto. Then finishing graduate studies at Columbia and psychiatry in
London (add two years each, since he's a comic book genius).

He then meets Jean prior to X-Men #1, at which point Jean is 10 and he's 26.


I suppose it's still a big enough difference to be creepy, but it's still nowhere near what it looks like. And I'm pretty sure all this backstory wasn't planned at the time of X-Men #1 and he could actually have been intended to be close to Jean's age.