The officer gives you a ticket and if you’re not an adult your parents pay the fine. It could also go further if a child with a report to protected services. But most likely just fines unless repeated.
Whether the officer acts or not is individual. But if the officer at a a good one they will be diligent..
As for youth using an adult’s ID to get around an age check? The UD owner would likely face consequences for not keeping their ID secure contributing to the diliquency of a minor if something happened to involve an investigation. However difficult to say.