I found something interesting in the anime.

[valfranx]

episode 196:

"7,000 years ago, Migoren fought valiantly with aliens.
3,500 years ago, State protected his planet by turning away a meteorite.
Meet Olive, who also came from earth, and is remembered as a legendary hero.
The others were expert fighters in their own lifetimes as well.
These martial arts experts have trained here for billions of years hoping to be trained by Dai Kaio-sama.
Eh? They keep training in hopes that one day they will be trained by the Dai Kaio-sama?
That is why only those good enough will have the honor of being trained by Dai Kaio-sama.
Is that so?
You still have to work hard in the Netherworld."

then it is as old as the age of our universe.

[Alestat]

He's talking about the Dai Kaios in general, not about one specific Dai Kaio (the metalhead old geezer).
The average lifespan of a Shinjin is 75,000 yrs. The ones who are in charge are granted a few million years extra (at least 5 millions).