My father was 10 years older than my mother, and they got married when she was 20. Needless to say, I never thought of him as a "dirty old man".

With regards to Xavier's age difference with respect to Jean, I'm guessing it has probably varied from writer to writer.

After she was first "killed" as the Phoenix, the headstone shown in X-Men #138 said 1956-1980, making Jean 24 when she "died".

In X-Men #161, Claremont had a flashback featuring Xavier and Magneto as adults set 20 years in the past. In that flashback, Xavier is a full fledged psychiatrist, treating holocaust survivors. That would suggest to me that Xavier's age is 40 or older in the then current continuity (X-Men #161 came out in 1982), making the age difference 15 years or more.