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.
Yeah, the vibe I get from the early X-Men issues is that Xavier was a fairly young man. Not straight out of school, necessarily, but a highly motivated young teacher who tended to come off older than his years because of his experiences and intelligence. Back in the early 60s I'm not sure a guy in his late 20s having feelings for a gorgeous 17-18 year old girl would have been viewed as so skeevy. Not to mention other out-dated sexual politics going on in other books (e.g., Daredevil, Iron Man, Thor). Maybe they should do a Mad Men style series about early Marvel