I was beat by a 13 year old who had no idea how to play chess i was older than him.
In 2001, when over 50, I played in the US Open and lost to a kid of about 10 or 11. He was higher rated than me (in the 2000s) and said in an interview with Chess Life Magazine that he really wasn't interested in chess but his dad pushed him into it.
This would be interesting to look up - all the archives are out there on uschess.org today.
John William Schulten - "one of the biggest losers in chess history" - Andrew Soltis, in his book David vs Goliath - How to Beat a Stronger Player
Apparently he was destroyed by practically every major Romantic era player.