Joke about professor, cheating vulgar

A college math professor and his wife are both 60 years old.
One evening the wife comes home and finds a note from her husband.
It says: "My dear, now that you are 60 years old, there are some things you no longer do for me. I am at the Holiday Inn with my 20-year-old student. Don't bother waiting up for me."
He returns home late that night