Minor Purchasing Alcohol PA (S 18 6308)

There is a world of difference between being charged with a crime and being convicted of one, particularly in the circumstances you've described.  Can the police charge you?  Certainly.  Can the authorities allege facts at an arraignment that would compel a judge to conclude that there is sufficient evidence that a crime may have been committed to hold you over for trial?  That's a roll of the dice.  As a practical matter, your most significant concern is probably whether the 21-year-old in your story is going to roll over on you if he is arrested. If your stories are absolutely consistent, and he confirms that the money he gave you was indeed money he owed you for lunch, the authorities are unlikely to push this all the way to the trial phase, but you can expect them to push you very hard to cop a plea.  Your best case scenario is one in which both of you say exactly the same thing if you are questioned separately and asked, for example, where you ate for lunch, what did you have, where is the receipt, and isn't it just a little too convenient that the change from the beer purchase was equivalent to what the 21-year-old owed you for lunch.