Rob the cradle
Definition 1 of 1
Pro Tip 1/3Slang Term
This phrase is informal slang and might not be understood in formal contexts. 
At a formal dinner, saying 'He's robbing the cradle' may be inappropriate.
Pro Tip 2/3Potential Disapproval
The phrase often implies a negative judgment or disapproval of the age difference. 
When I said he was robbing the cradle, I noticed some people frowned.
Pro Tip 3/3Gender Neutral
Though typically used for older men and younger women, it can apply to any gender combination. 
Sara is only 22 and dating a 50-year-old; some say she's robbing the cradle.