I guess they are highschoolers
They're 17
According to OX's early chapters jian yi gets kidnapped and sent to an island the second day of high-school and then comes back when they're adults so they're actually still in middle school.
Are middle schoolers 17 in China? I know in some other countries middle school is 3 years. But wouldn't that make them like 15 or 16?
No, they're high schooler. In China, high school is called "中学", which mean "middle school" in english, but it's actually high school.
Its so weird to remember that these are all middle school boys (I've never found them "hot" for that exact reason, although I do think they're all adorable and I love to see their relationships blossom). The author draws them like adults now and not that I'm complaining but it does make it confusing for people to understand the time line (It's properly explained in the chapters that are in the future.)