I get what you mean, raising a child is hard, I know from experience. you have to want to put the effort in. However, I don't think his mental age is fifteen, I just think he's ignorant to relationships, lol. Since he never experienced what it was like being an omega even though he is biologically one, and now all of a sudden experiencing it while having an Alpha trying to help him while wanting to be in a relationship. The guy's ignorant, not dumb, although some of his thought processes are. Either way, I'm sure we will see a baby later when they finally decide to be together and we just left with the brother's relationship problems.
A 26 year old adult would not need to come up with a good lie or excuse not to move back home. Doyun's doctor says Doyun is still a child in many ways. Doyun's defensive behavior at the coffee bar has teen mentality written all over it. He needs to be called "hyung" by his Alpha. Big brother is constantly worried over Doyun's ability to live on his own.
That doesn't mean he's a child -_- that just immaturity due to inexperience. I know the difference personally between an immature person and actual mentally challenged individuals with the mentality of a child. I deal with them every day in my personal life. Which is why I said he's inexperienced, not mentally retarded. If he was, he wouldn't even be holding the kind of job he had, let alone would the doctor allow him to even have relations because of his medical problem. However, the doctor meant it not in a literal way, but in a metaphorical way, as in an inexperienced way. You don't as a person have a literal metamorph from baby, to a child, to teen, to adult, to elderly. As a baby you grow and learn which you bring with you as you grow into a child by then you don't remember your babyhood, but what you learn is there and you don't even realize it but you know things you don't remember being taught. Same as a child, you experience things and as you grow into a teen those memories go with you as you experience. If someone doesn't experience relationships or social settings as bars and such when they are young, you would obviously be inexperienced when you are older. Doyun hadn't experienced relationships romantically when he was young, and certain settings are unknown to him causing him to act in certain ways. So, of course he will come across as mentally a teen, or mentally a child to someone who has more experience in these things. So, don't write others as mentally retarded just on the basis of their inexperiences, it's not that they are dumb, but that they do not have the knowledge that someone who does, have. Also, Doyun's older brother is worried about him, because he is his brother. Not all families hate each other, and not all siblings can care less about their younger one's ability to live. If they have a close relationship, of course, regardless of a problem, the older and younger would generally care about the other's ability to live a successful and good life regardless. Doyun's wants to be called "Hyung" because it is his own little way of trying to lie to himself, and also his kink. Doyun knows secretly that he loves his alpha and that he wants a relationship, however, due to his inexperience and fear of the unknown he isn't ready yet to take that leap. I can keep on explaining the psychology and these character's whole settings, but I don't want to write a research paper. In short, experience makes people mature, and inexperiences make us immature. It's how humans learn, how they work, and how they think. oh, and you can disagree, nobody is wrong here, believe what you want, I'm not gonna try and change people's opinions, I find other's opinions very interesting.
Yeah Doyun is immature with regard to romantic relationships.
But I think he asks to be called "Hyung" not only because of his kink.
He is the baby of his siblings, the younger child and by being a hyung to someone else makes him feel like the bigger person for once.
I still find him such a cutie (●'◡'●)

Doyun's mental age is probably about fifteen right now. To want him to get pregnant and have babies seems to me a cruel desire on the part of some readers here.
Research shows that having babies doesn't make people happy. Though marriage and pregnancy might be what happens here, I don't think that wishing it on these characters is appropriate. I hope that's not where the story is going.