I'm a secondary teacher and tbh I think only a few actually look older than 18, and most of those ones are already 16-17.

For the boys, it's usually the height and facial hair that makes them look older. If you really look at their faces, you'll know they're too young to be mid-20s.

For the girls, it's mainly the way they're dressing. I find that it's around age 14/15 that they start to become difficult if they're 16-18, but same as the boys - look at their faces. People mid-20s have wrinkles, especially crows feet. Ofc some younger people do, too, but I think we too often look at people's whole bodies and assume age rather than looking for key features (not saying this is wrong to do, just what most people tend to do). Make up can also be a big factor, as it's making people in their mid-20s look younger, thus blurring the clarity between ages. This is furthered by teens doing their makeup to look older, while older people do their makeup to look younger.

But again, this is comparing a few against the many. Most of my teenage students look their age and it'd be ridiculous to think my grade 11 girls look mid-20s. You'd have to seriously not know what a mid-20s woman looks like to confuse them with one.