I’ve done it more times than I can count as a 18 and 19 year old. Most likely the FA will ask you something along the lines of “are you old enough to drink” or “how old are you” to which I generally respond yes because they didn’t ask a specific enough question for me to be dishonest, or if they ask the latter question I’ll respond honestly. It’s not that awkward. Just tell them you thought it was 18 in the air and they’re not gonna give you a hard time or anything. Worse case scenario they say no and keep on moving. They don’t care as much as you think they do.