Hakanai Kimi wa Moukou wo Hajimeru

[danvolodar]

Aight, I'm gonna address these points one by one, even though my comment is like a year old and I only partially remember what I read.

The main characters are not dating, neither has one of them adopted the other. The five-year age gap is there to stay, however.
There is nothing wrong with people who knew each other as kids to get together when they grow up, even if there's an age gap. Everyone was a child once. Actually, it's perfectly fine for an adult to get together with someone they knew as a child, when the child grows up, unless the adult has been grooming them.
The story basically ends by ch.25. It's overwhelmingly just page-long arts after that - the mangaka fell seriously ill, from what I gather.