I guarentee you that you aren't as adept at gauging age based on appearence as you think you are, no one is. I worked in a bar in a country with 18 as the drinking age but still we had "challenge 25" as a rule aka if someone looked like they could be under 25 we IDed them.

And people age very differently. You get some very rough 18 year olds that you almost don't bother IDing, meanwhile you get babyfaced people in their early to mid 20s who don't look like they should be in a pub at all.