You can remain silent, but the walking away might be difficult. The officer is allowed to temporarily detain you if he has an articulable suspicion that a crime has been committed. If that rises to probable cause, he can arrest/charge the person. Since the standard for those under 21 is that they can not consume *ANY* amount of alcohol. It doesn't even take a breathalizer to convict.

However, you've missed the point of this board. This is for people who need legal assistance, rather than the "How do I break the law and get away with it" forum.