dirty old man

(idiomatic) An adult male - usually middle-aged or elderly - who acts in a lecherous or lewd manner.

Example:   The young women in the bar turned their heads in disgust when that dirty old man began to wink and smile at them.
1969, The Beatles, Mean Mr Mustard:
  Always shouts out something obscene / Such a dirty old man.
1991, Robert M. Pirsig, Lila:
  If you didn't go for Lila you're some kind of prissy old prude. If you did go for her you were some kind of dirty old man.