American, humorous someone who has a romantic or sexual relationship with a much younger partner.
He's a cradle-robber. He married a 16 year-old and he's nearly 30!
cradle-robber
A jocular term for someone who is romantically involved with a much younger person. I wouldn't have pegged Jeff as a cradle-robber, but I just saw him around town with a girl who looks like she's barely out of high school.