The standard US Customs procedure when finding underage persons in possession of alcohol is to direct them to a nice friendly sink and politely ask said underage person if they are sure they would not like to voluntarily dispose of said alcohol themselves. Problem solved. Of course said person is free to not volunteer to dispose of the alcohol. But that of course would set in motion other remedies.

This knowledge was gained from great personal effort some years ago testing US Customs policy on under aged people with alcohol. In fact every possible loophole I could think of was tested. Some quite creative and clever. But none worked other than the fact that usually they don't look in your bag anyway. The policy towards middle aged adults with more than a liter of alcohol is remarkably similar.