I am 19 (soon to be 20) and have been served drinks several times. Even when I was 17 they offered, but sometimes I was scared and sometimes I was a litte braver to say yes. I've never been asked to provide any ID and don't think it is necessay. I don't think it is a big deal, even though against the law, to be served in the air b/c the FAs have control on how much you consume.