I went at 20 with my family.
My oldest brother was 24, the next 22, me 20.
I used the oldest's ID.
We went to bars and lined up in this order.
Me, middle brother, random joe, random joe, oldest brother. I used his passport while he used his ID. No clue.
We sat at a table playing blackjack. All three of us. I started doing really well. Looking youngish and winning money the pit boss decided to come ID us all. We're the only three at the table. Went from Oldest, to middle, to me, two of us showing the same persons ID. They didn't notice.
So to answer your question - no idea haha!
But they don't seem smart enough to care, so I wouldn't worry about it. I think you'd just get booted from the joint. make sure that's the only ID you have on you though. That way you can demand it back so you don't lose it. If they don't believe you they can still boot you, but keep your ID. It'd suck to lose it the first night