That kid isn't 30 years old. He didn't live in Japan. He merely inherited the memories of someone who did, but it wasn't he who lived there. He was already a person of his own, who had gone through a lot more than any kid his age should have, when he got those memories. That makes them forever external memories and not a part of his core personality, even if they are a part of his identity now. But he hasn't lived that Japanese dude's teenage life plus then his own. He's only as old as he is in this world.
It would be a different thing if he had been born with those memories and thus wouldn't ever have been an infant and child in this new world at all. But in that case he wouldn't have gone through all of this either, I reckon. As much as the Japanese love slavery (based on isekai where 90% of Japanese MCs don't hesitate to acquire slaves), I doubt a Japanese person would volunteer himself to as dreadful form of slavery as being sent to the mines (which in real history was pretty much a slow/delayed death sentence). But since Reiji (who wasn't even using that Japanese name back then) isn't a Japanese person, he did it, not knowing any better.
Of course his soul is "contaminated" by the original Reiji's, so that he got twice the number of orb slots and even Japanese looks. But that's just external factors, once again. They don't define who he is, although for sure they have affected greatly his life. As to why he insists otherwise, that's not difficult to understand: his life has been so shitty that the Japanese Reiji's life was like a paradise in comparison, so he kind of flees to that paradise, substituting his own past with Reiji's past. Yet it's also revealing in practical matters Japanese Reiji's past doesn't really matter to him: His goals in life are related to his own past, like finding Master Hunger's grandchild and Lark.