Dude, I sold beer to a 13 year old years ago when I was busy in a retail store. Turns out it was a sting. I got busted. They let me off with a warning. Thank goodness!! I even glanced at her ID first!!!!!
So, don't feel bad about this one at all. These jobs are difficult and people who do things like pass fake bills know to come in when you are already busy and distracted.