Third world countries usually don't mind young Americans. Argentina has been repeatedly asking to get back into the Visa Waiver Program. Perhaps they're turning more Americans away in hopes that the State Department will approve Argentina for the program. It's idiotic, but as someone who spent 15 years working and vacationing in Latin America, idiocy is standard operating procedure.
It's always a good idea to have your hotel info printed out. They theoretically could deny you entry because of that.
The kid could have said something stupid, or refused to do something they asked him to do.