I am 24 but I look and sound much younger. I am really short and if I am in jeans and a tshirt, I can pass for anything from 14-18, but rarely for my own age.
Last summer I was at a kind of touristy spot in P-town in MA and was looking at various knives and such to get a gift for a friend. The cabinet had a big ASK FOR ASSISTANCE sign, so I went over to an employee and asked to be shown a particular knife. She flat out told me "I can't do that." I assumed she meant she wasn't allowed or something, so I asked for someone who could. She then said "Look kid, don't even try it." At which point I told her I was 23 and went to get my ID. She told me she "wasn't taking my shit" and walked off before I could get my ID. After I took it out, I went over to a cashier and asked for the manager. I told the manager what happened, showed him my ID and he told me "That's a fake ID, get out of here."
it was my US PASSPORT which CLEARLY SHOWED MY DATE OF BIRTH (1984). I understand they probably have a ton of crappy kids coming in and being annoying but there was no excuse for treating me that way. He hardly even glanced at my ID.
Obviously I didn't give them my money, and ended up buying a rather snazzy blade at another shop.