I'm 31, usually have people guess early twenties, but have very occasionally had people still assume I'm a teenager (not very often though and personally I don't think I look under 18). I got people ask where I went to school until I was well into my 20s.
Oddly I rarely get ID'd for alcohol but do get ID'd for all sorts of other things, and have been refused buying matches because I didn't have ID. I hate how arbitrary requiring ID is for age restricted products is in the UK, for a lot of things they're meant to ID you if you look under 25 (even though the age restriction is usually 18) and it's all dependent on the person serving so there's no way there can be any consistency in that judgement. I've got proper annoyed on several occasions where I've gone all the way back home to get my ID, come back, and had another staff member not check at all. I think on one occasion, I had an argument with the cashier who wouldn't let me buy butane canisters (I was 28), stomped back home really quite upset about it (I'd been in the same shop numerous times to buy the same thing), got my ID, walked back again, and I think the same woman didn't ask me for my ID the second time. I don't mind getting asked for ID but the inconsistency of it all drives me crazy.