That's not true in all states, and could really be true in - at last count - no more than 18 (including the OP's of NY), depending on the circumstances.
However, given that this is inside a bar, there are other potential theories that might apply - especially if the bar checks IDs at the door. But, in this hypothetical, WHY would the bar suddenly get raided?! Unless he intends to attend a bar where they are soliciting paid sex acts, or doing dope on the tabletops, I doubt the cops would raid the place like it was a speakeasy of a bygone era. Though, the police might do occasional ID checks inside a bar, and in such an instance, he may be required to present his ID to show he is 21 or older.

