Did they say you can't play a game rated M? If they did that sucks... I'm 15 and most of my games are rated M, but I've been playing rated M games since Conker's Bad Fur Day on the Nintendo 64 (when I was10-ish). You could try sneaking it by asking for it and not mentioning that it's rated M. You could just ask if you can buy a rated M game and show them why it's rated M (blood, language, etc.). If it's something small, maybe they won't mind.