Rules vary. In Britain it was completely normal for say 15-year old me to be in a bar - it was illegal to buy booze but not a problem to be there. But when I travelled to Austin aged 19 I couldn't meet adult members of my team in the hotel bar because I wasn't old enough even though by then I was legal to drink, to marry, to go to war and so on in my own country.

A little while after that, back in the UK, I drove my young cousin to the seaside. I didn't carry ID - I don't drink and you're not required to carry ID to drive here† so it was never necessary back then, but she did, so I try to buy her booze, they demand ID, I do not have any ID so I can't buy it even though I'm old enough to drink. So, she just orders her own booze, she's under age but they don't ask because she's pretty.

† The law here says police are allowed to ask to see a driving license if you're in charge of a vehicle on a public road, but, since you aren't required to carry it they can require you to attend a police station and show documents within a few days. In practice in 2026 police have network access and so they can very easily go from "Jim Smith, NW1A 4DQ" to a photo and confirmation that you're licensed to drive a bus or whatever if you are co-operative.