@ManhwaAddict It's not mentioned yet but here's what we know so far
1. He is a university student, but younger than Haesoo (who is also in university)
2. Eui-hyeon describes Yoonhwa to be "around the same age as him". Yoonhwa has also been shown to be of university-age.
2. He (eventually) speaks in informal speech with Soo-a and Soo-a doesn't seem to use any honorifics while referring to Eui-hyeon.
Therefore I conclude that he should be at his late teens/very early twenties at the maximum. If we assume that he's exactly the same age as Soo-a and Yoonhwa, that'd make him around 18.