We don't have an exact figure to put on it, but we're pretty sure Charles Francis Xavier has been around the block a few times. Marvel has this slightly confusing sliding timeline which means that characters who were introduced as teens or twenty-year-olds in the sixties aren't eligible for a bus pass now.
Peter Parker has slowly developed from high schooler through college to being an adult, when actually Spider-Man should be carting around a zimmer frame by now. Originally it was Frank Castle's tour in Vietnam that gave him the skills to be The Punisher, but nowadays it's the first Gulf War (or maybe even the second), so that he's not an octogenarian going around murdering criminals.
The Professor X we see in modern-day comics looks to be in his forties or fifties which, considering he looked about the same when the first issue of X-Men debuted in 1963, would make him like a hundred years old now. Which isn't quite right, but even with that sliding timeline thingy he's pretty old.
After all, he needed time to get a bachelor's degree in biology, study for a master's at Cambridge, sleep with a load of women who thought bald was sexy, and had a load of adventures before getting crippled and starting up his School for Gifted Youngsters. Including hanging out in Israel with one Erik Magnus Lehnsherr, who was a kid during the Second World War, making both him and Charles at least sixty or seventy years old.