One of the reasons, in my mind, that 13 year old Damian and 10-year-old Jon just would work better is that Damian really has never had a childhood. He tries to come off as mature and adult, but deep down there is a part of him that's pretty vulnerable and sensitive. There's a part of him that is envious of everything that Jon has: the well-adjusted childhood and the emotionally available parents. While Damian wants people to believe that emotionally he is 25, in reality, deep down, he's a little kid. Younger than Jon in some ways. He takes comfort in animals, he's stubborn as hell, his most fulfilling relationship is with a boy 3 years younger than him. And regardless of whether he would admit it, it was Damian who really sought out Jon's companionship, because he almost needs Jon more than Jon needs him.
It was an incredibly rich, interesting, and complex dynamic that revealed a lot about both characters and now all that potential with them as they were might go unfulfilled. That just sucks.