cradle snatching 

This is when you date someone way younger than you.

The 21 year old was cradle snatching a 14 year old

cradle snatching by MrInsest May 2, 2016

Cradle Snacking 

Variant on the common term ‘cradle snatching’ for someone who is sweet on a much younger honey. If you are not in it for the long haul but simply want to sample the youth for a short while, you are ‘cradle snacking’.

Don't worry, I'm only cradle snacking

Cradle Snacking by bonesofthewhitegoods July 7, 2010