It's not your age, I don't think. It's that you've never experienced this specific thing and can't relate. If anything, Uehara is acting far older than his age here, because he's able to shrug off something like this with "what could I expect" instead of the identity killing knife it would be to someone like him. Trying hard, doing something big, being sure it's big and important and then people mostly ignoring it or telling you they don't like it is how kids give up on things. Personally, this is my interpretation of his tears; the deep sadness of thinking he should just give up because nobody cares about what he makes being flipped on its head by Niina personally, undeniably, showing him that someone does.
The manga's been good the whole time, but these past 3 are incredible. The mirroring between the beginning of chapter 31 and the end of it, the mirroring between chapter 32 when Uehara is there for Niina and chapter 33 where she's there for him. The way 32 shows and expresses the idea of being with someone who makes you think you can do anything. How 33 effortlessly ties the idea of her love and appreciation of his work with her love and appreciation of him. They didn't even fucking have to SAY IT it's just THERE
Like this shit's getting out of control man