This is from "Personal Experience" and not a "Set Rule" so don't shoot me if I'm wrong... ( or you get the "wrong" bartender)
I've gone to JA, with teens that were "almost 18" and they had no problems getting served at an AI. My Son was 9, last time out, and they would give him a beer for me.. ( I need a red-slushie, and a beer for my Dad... )
Quite often, at check-in the "parents" of the "almost" were asked if they wanted a adult/child wristband for the person in question.. (wink, wink)
All I will say, is : The staff are doing a courtesy for you if they allow your "minor" to drink, without you having to run to the bar each time, so PLEASE be responsible for your "little" ones...
One bad News Report on CNN, about an Accident; and it will be 3 pieces of ID.. to get a drink..