
It is a BIG DEAL in this situation because Jaeha practically witnessed how Eunho grew up. The author also mentioned this on twitter. Zec also said, that's why Eunho is so desperate. Because Eunho wanted Jaeha to see him as a "man" rather than a kid brother.

it is for some people, so we need to separate what we think from what character would think. and you didn't understand what they wanted to say, it's a big deal for him because he watched him grow so he has always seen him as a kid, and all of a sudden he tries to prove him that he's a grown up and a man. I'm a bit annoyed by Jaeha, but I get him in this situation.

well I get it in a way, since he kind of was his big bro. but he still wasn't that young, they make him seem like he's 2 years old or something XD and what to do, people do grow up, even if he did know him has a child, he need to get that people do "grow up" in the world. I think it still his fault for not getting it. I don't blame him for not loving him back right away or anything, I just think he need to get that people DO grow up. so what if he know him has a kid for a long time, 21 is NOT a child for sure XDD
and I did get his feeling when he confess to him has a child, no one will take it seriously. it true XD cause he was a child at the time, but he 21 now and still love him. I think it very impressive and cute.
Jaeha and Eunho not that far in age, like I don't see the big deal of 8 years. I seen couples that where 10 and even 12 years apart and the mange don't seem to have any problam with that.
Jaeha is 29 and Eunho is 21, like no he's not a kid at all. 21 is already an adult, so this manhwa make a big deal out of nothing, really. there is girls that 21 that going out with much older man then they are, but they still in their age group, their 20-30. so I don't see the big deal, I guess gay it's "bad" in this or something. cause I bet if he was a girl, Jaeha had no problam with his age.
http://www.mangago.zone/read-manga/lover_boy_lezhin/uu/lover_boy_lezhin-chapter-25.5/3/
he's not that young, he really isn't XDDD