;/

Yes, it's quite odd how he is drawn like a 10 year old kid. I would assume early twenties seeing as he apparently has been and then dropped out of university and being working since then?

In "If you hate me that much" he is the same age as Hyung, so early 30's.

You are referring to Hyeseong. Yes, he is drawn very young. But he used to look much older in the original series:
http://www.mangago.zone/read-manga/if_you_hate_me_that_much/uu/if_you_hate_me_that_much-chapter-7/10/

Then he started to change:
http://www.mangago.zone/read-manga/if_you_hate_me_that_much/uu/if_you_hate_me_that_much-chapter-20/16/

And by the end he looks like this:
http://www.mangago.zone/read-manga/if_you_hate_me_that_much/uu/if_you_hate_me_that_much-chapter-37/10/

So in the same manhwa he went through a lot of changes.

Yes, he looks and acts completely differently now. In that manga he is 24 years old, so why the author has chosen to make him look and act like someone more than 10 years younger is a bit weird especially given the subject matter.