How old does he look? Seriously comics are notoriously vague about such things....If the characters we read about were to age chronologically as we do, pretty much all of our favorite characters would be elderly...Spider-man was first introduced 50 years ago...if he was supposed to be around 20 then that means he'd be in his 70's right now. My point being, Superhero's in comic books are pretty much perpetually the same age. Xavier has always been portrayed as being a bit older than the Mutants in his school...So I'd put him in his Mid to Early 50's...No younger than late 40's.
The Marvel Universe has a sliding timescale, so its often guesses that everything thats happened from the forming of the Fantastic Four/ Avengers/ X-men to lets say the Schism between the X-men, death and return of the Human Torch, Seige of Asgard ect has been around 15-20 years, so i'd guess he's in his late 40's-mid 50's