I think its it's because he is an omega and in the omega verse omegas are usually small and cute, the opposite of alphas. I can see where he looks like a child to you but it really just the aspect of how omegas look.
There are men who actually look like Hye-sung through.
Look at the couple in this video.
https://www.pornhub.com/view_video.php?viewkey=ph5d2228e29d1a4
The bottom has a body like Hye-sung and the top has Dojin body so... there are men over 20s who still look like teenagers.
ლ(´ڡ`ლ)
if you haven't seen the original webtoon where this one came from, hyesung and dojin actually were closer both in appearance and age, but i guess the author changed that in order to cater to omegaverse designs. he is actually more worldly and less childish in the original.
I will always have a problem with the way Hyesung is drawn tbh. Did he really have to look like a kid ? It makes the sex scenes look disturbing to me.