Although I could substitute 'identify' in place of 'ID', would it be considered improper to use 'ID' as a verb? Here's what I mean:
That boy looks underage, I think we should ?ID him.
It's in Merriam-Webster as a synonym of identify, and certainly most people would know what you meant if you said it. It comes across as a little informal and reminiscent of police jargon. If you're talking about IDing someone specifically to verify his age (as in your example), you may also hear the colloquialism to card.