In UK, if someone is dating a much younger person, they might be called a 'cradle snatcher'. It is mildly derogatory.

Cradle robbing is a slang way of saying that someone is dating someone much younger than themselves. Example: Tom is robbing the cradle dating Kim. In this Kim would be younger than Tom. Example: John said, "There is nothing wrong with cradle robbing." I would say that it is more common to hear of someone 'robbing the cradle' than 'cradle robbing'. I hope this is useful!