rob the cradle

To have a love interest that is significantly younger than oneself in age.

antonym=rob the grave

Typically, older man and younger women, but not always.
"I want to rob the cradle, I want to trade my 40 year old wife in for two twenty year olds"
For example, further than 5 years apart in age might signal a cradle-robbing". Even more years yells it!

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