I don't think so when you bring out the age as an excuse for being an asshole it's weak I mean at that age people know what they are doing, there's not excuse for their behavior if they are insensitive it's because they want and they are aware of that. Jin was wrong when he put his expectations on hamura without telling him why if it really was a mayor reason, But the way hamura answer was over the top and so rude I can't stand when people are jerks just because they can, besides what I suspect it's that hamura had never saw his relationship with Jin like a boyfriend to boyfriend relationship it seems like Jin did so so probably hamura broke Jin's heart more than once that's just speculative tough
First I'm not sure anyone is being an "asshole". The importance of age here is that they are both young and in high-school when all this happening and how they both understand what's going on. Jin clearly takes it much more seriously and sees long term perspectives, while Hamura doesn't believe it can last beyond them being separated after HS, so takes it less emotionally serious. I doubt either of them would have discussed it, but would both have assumed the other understood what each thought was a given. If Jin's heart was broken he half did it himself, as all Hamura has basically said is that he isn't sure what love is and that he doesn't believe in long distance relationships. But most 17 yr olds tend to be rather self-involved, paying more attention to themselves and their own feelings and not as much to those of others, even those they care about. At that age you often have to spell it out for them. When they grow older by just a few years they get over this phase. So yes, age matters.
Since I haven't looked at any raws my comments are going to stick to what has actually been scanlated. I know Hamura seemed unfeeling and self-centered, and yes he was a bit, but you could also say he was being rather realistic at the same time. How many long distance relationships survive if people can't see each other? Especially if they're just kids when life separates them? And how much does any teen really know about love? Isn't normally more often a case of attraction mixed up with whole idea of wanting and having a BF/GF? I'm not saying there's not exceptions to both, couples who manage to keep together despite distance, and teens who really do love. Hamura's attitude is more real and perhaps even honest (since it at least admits there might be problems ahead) than Jin's which is more ideal. However, Hamura is also insensitive to Jin's feelings, but to be fair he is only supposed to be a 17-18 yr old at this point. But personally I have a hard time imagining a kid of that age with Hamura's POV really being able to understand Jin's more idealistic POV, and the same for Jin understanding Hamura, until they're adults, which might mean they're likely to go separate ways when young from the get go.