I don’t know about “fool around”; the youngest person of interest is twenty, but she’s a sweet little mess, so my “fooling around” with her mainly amounts to “stop letting boys push you around,” “quit wasting money,” and “eat something other than candy”. And telling her she’s pretty when naked and/or crying.
The youngest idiot who’s fully involved with me is Peanut. She first scratched at my door at nineteen, but didn’t make it inside until twenty-five. The nineteen year old version was useless to me… she reeked of entitlement, and expected me to be grateful for the chance to plunder her overvalued youth. The twenty-five year old version is —at long last— useful… she’s scrubbed the smug from herself, and she finally knows which way the gratitude flows.