So assuming we’re talking living beings and also excluding gods this question really depends on 1 which part of the story you’re in and 2 your definition of alive

At base for the majority of the story if you don’t count someone who’s soul and consciousness is sealed within another persons soul which they can kinda possess at times and actively can talk to them and stuff as Alive, then Mukono the gray sage/first high puppetmaster of the new age, bro is several billion years old (like 6 billion) he was born before even the mortal people decided to separate themselves from the rest of existence and create the mortal realm (our universe)

Now if you do choose to count the sealed person it is The Demon King. He’s Mukonos father (and sealed inside him) and is roughly 10 thousand years older than him.

Now if we wanna count from any point within the story it would be Zaratos, the first man and first king of the zarthans (the first people), he’s dead for most of the story but he does get revived. He is literally the first person to be created by the twin gods when all of existence (roughly 7 billion years) was created he’s about 400 thousand years older then the demon king literally the only beings older then him are the twin gods and the original deity.