Hey ryder,
Sounds like your cousin should be okay, as the legal drinking age in Spain is 18, according to
wikipedia. The probable reason there seems to be so much confusion about the actual age is because drinking laws in Spain are not handled the same as they are in the States. It's much more relaxed, and I've heard certain regions of Spain lowered their age to 16. I would assume that even if your cousin was not 18, he would not meet much resistance, so long as he isn't out there causing a scene. That could possibly cause them to make an exception ;)