Hey BiAggieGuy,
totally agree if it happened like you described, but honestly I doubt that's how things went. I was wondering what GRINDR's guidelines concerning age look like exactly and found this on their homepage:
1. AGE RESTRICTION. THE GRINDR SERVICES ARE AVAILABLE FOR INDIVIDUALS AGED 18 YEARS OR OLDER.
1. AGE RESTRICTED ACCESS. NO PERSONS UNDER THE AGE OF EIGHTEEN (18) YEARS (TWENTY-ONE (21) YEARS IN PLACES WHERE EIGHTEEN (18) YEARS IS NOT THE AGE OF MAJORITY) MAY DIRECTLY OR INDIRECTLY VIEW, POSSESS OR OTHERWISE USE THE GRINDR SERVICES.
2. AFFIRMATION OF CURRENT ADULT STATUS. YOU HEREBY AFFIRM AND WARRANT THAT YOU ARE CURRENTLY OVER THE AGE OF EIGHTEEN (18) YEARS (TWENTY-ONE (21) YEARS IN PLACES WHERE EIGHTEEN (18) YEARS IS NOT THE AGE OF MAJORITY) AND YOU ARE CAPABLE OF LAWFULLY ENTERING INTO AND PERFORMING ALL THE OBLIGATIONS SET FORTH IN THIS AGREEMENT.
So he had to assume the boy was of legal age until told otherwise, and the kid most probably didn't run around screaming "I'm fifteen, please fuck me!" And as you stated yourself, it's not easy to tell nowadays: there've always been boys who look younger than they actually are, but more and more boys mature (at least physically) earlier than they used to do lets say 30 years ago. Don't get me wrong, I haven't seen any picture of that kid (and for his sake I hope we'll never find one in the media), but if a guy claims to be 18 and looks like it, a lot of us wouldn't bother to ask for his ID, don't you think?