I didn't get carded the FIRST time I ever bought liquor. Hell I still don't get carded half the time and I know I still look like I'm 16...maybe 17 if I haven't shaved in a while.
I honestly think they just assume I'm older because no one with my face would even bother trying a fake.
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http://soundcloud.com/thundersheep/lunatic-asylum
http://soundcloud.com/thundersheep/summer-rain