In the real world, most people won't live into their hundreds, and those who do aren't going to get too far into them - and they certainly won't live to be 200, 500, 1,000, 10,000, or 3 1/2 trillion years old while still looking like a teenager or a twenty-something. Anime characters who are hundreds of years old might defy reality, but that's part of what makes them so much fun.
Among these super old anime characters are characters like Master Roshi from Dragon Ball and Tomiko Asahina from From The New World who are able to intentionally extend their lifespans through magical means. But there are also characters like Joseph from The Ancient Magus' Bride and C.C. from Code Geass who have been living for centuries despite fervently wishing for the end of their endless lives.
Each of these characters offer a unique perspective on what it's like to outlive everyone they've known and have a history that spans generations. Which ones are your favorites?
3,000 years ago, Meliodas was next in line to become the Demon King. Despite the fact that a lot of people's hopes were riding on him doing this, he ends up prioritizing his relationship with a goddess named Elizabeth instead. Rather than simply take his life as punishment, the Demon King and the Supreme Deity worked together to punish them both for their insubordination. Meliodas is cursed with immortality - every time he loses his life, he's brought back again. In each life, he meets a reincarnation of Elizabeth, whose death he's ultimately forced to endure. That's pretty harsh, especially when it's over the course of thousands of years.
Senku may look like a teenage boy, but he's actually been alive for over 3,700 years. We'll focus on Senku since he's the main character, but this also applies to Taiju, Tsukasa, Yuzuriya, and many of the other characters. Senku didn't accomplish this through any special powers of his own. Rather, he was encased in stone along with the rest of the world, and was only freed after 3,700 years. However, his unusual intelligence is what allows him to survive once he breaks free of the stone - he's able to figure out how to gather food, build shelter, and begin the process of recreating all the tools that science has come up with over the course of millennia. Were he someone else, he might not have lasted long.
Yato isn't completely immortal - he can be taken out by supernatural means. But things like poison, explosions, and gunshot wounds that would destroy most people have little to no impact on him. That's because he's a god who has been alive for 1,000+ years. Specifically, he used to be the god of calamity, sowing destruction and pain wherever he went - but he decided to clean up his act and become a 'delivery god' instead. While this earned him the ire of some of his fellow gods, he becomes more respectable to himself.
While he isn't exactly immortal, Alucard is a startling 598 years old. He's a vampire, which explains his longevity. When he consumes someone else's blood, he also consumes their soul. This not only allows him to heal any injuries he might have accrued, it also literally adds years onto his life.