I get what you mean. It's hard to sometimes look at a character and remember that they are a legal and consenting adult lol. But I think that's the point. Hyesung looks young and small, he has characteristics that alphas typically don't have and that's why he was so proud of being an alpha. But now that he knows he's an omega, that appearance is probably a complex of his.
Neither of them really look like adults in the first place
LOL I was trying to be optimistic but that's true. I wish real life was like that. Every day I age about a month in appearance ahaha.
It bothers me that the uke looks like a child. It spoils the sex scenes for me... sometimes (=・ω・=)