Different countries often have different age groups under the terms "middle school" and "high school" than you might have had. The example I'm familiar with is Japan. They only have three years of high school, so that's about 16-18 for the ages. Middle school is until 15 or so in that case, which I'm pretty sure the MC said he was that age around the beginning of this.
Another matter to consider however is that people in Korea are counting their ages differently than Westerners to begin with. You're a 1 year old the day you're born, not a year later. So unless the translator already adjusted for that, by my standards the MC is likely 14 at the moment.
As for the level of physical development, it's a comic from part of Asia. Can you honestly say this is the first time you saw kids this age that looked like that in such a thing? The 12 year olds and under all look like they're about 5-8, and anyone 13-18 either look like they could be 16-20 something, or they look like an actual 12 or 13 year old while everyone else their age still looks 16-20 something because they're that one underdeveloped and flat as board girl who will look that way until she's old enough to develop wrinkles.