It was first brought to his attention by his sister, she now entering her 60s and he mid 50s, that while she had signs of her age he had none of his though they were so close in age. She had begun to show in her late 40s. Even his friends, Okubo and the other human two, had grown old and they along with Shizuru eventually died.
Kuwabara Kazuma however, still looked the same as he had back in his mid 20s.
And so it came to pass that Kuwabara Kazuma was dubbed "immortal" and officially took over control of the shrine and now demon sanctuary of the late Master Genkai. His ownership was proven when he wed the lovely koorime who had lived on that land for the past century, and cared for it the 80-some years she had owned the land. Together they protected the demons from the humans around, and even humans from the occasional demon attack. All the while entertaining the mutual friends who often came to visit.
Hiei who had left for Makai as soon as he could. Kurama who kept human form yet lived in Makai after the death of his beloved family. And Yuusuke, who had lived with Keiko and run her family's shop until her death. Then he vanished into Makai to take back control of his father's land from Hokushin. Raizen's friends came and went for the Demon World Tournament whenever they desired some further practice in battle, Yomi reamained lost in training with his son and Mukuro, content with life, never returned to her maximum power. Therefore the title or ruler was left to those of the next power level to take. Neither Hiei nor Kurama bothered with it either.
Things were peaceful though. The new government, though it did still have a few flaws, was overall successful in keeping the peace, and more importantly keeping demons (en masse) from the Human World. And the visits from friends were always nice. After several centuries though Kuwabara began to finally notice signs of age in his friends, subtle though they were. It was about the same time he and his wife Yukina both also began to grow older.
He had always been curious about it, why he wasn't aging. Sure everyone else he knew was curious too but they hadn't been around to hear all the dismissed theories. Of course they had first gone to Koenma to see, Yuusuke coming because he'd been forced to show the way. The Spirit Prince could only say that his date of death had vanished from the books, but they knew that much since the Ankoku Bujutsukai. He suggested perhaps it was due to the influence of his friends' demon auras on him but after Keiko's death that theory had crumbled as well. Searching into his past showed nothing that could have the effect of immortality and so it had fallen to be a mystery
Somehow though, he never noticed his friends' shared expressions everytime the matter came up.