This is always a touchy subject. When a girl is 18, any age difference is acceptable, but when she's 16, dating a guy that's more than 3 years older probably isn't a great idea.
Unless if the guy is a rock star, of course...
When Brian Wilson of the Beach Boys started dating his future wife, he was 21 and she was 14. They married 2 years later.
Elvis Presley met Priscilla when she was 14 and she moved in with him when she was 17 (even though she claimed they never had sex until the day they married - their only daughter was born EXACTLY 9 months later).
Jerry Lee Lewis married his 3rd wife when he was 23 and she was 13 (she was the daughter of his cousin/bass player)
Bill Wyman of the Rolling Stones started dating Mandy Smith when he was 47 and she was 13 (with her mother's blessing). His son later married her mother.
Chuck Berry was once arrested because he had a 14 year old girl working for him as a prostitute.