He’s the second oldest of the brothers, Death obviously being around a lot longer than his three siblings. Pestilence and Famine came into being when humans were created, their particular skill set not needed on the celestial plane.
He is exactly the same age as the archangel Lucifer, called into being (as a concept rather than a full blown horseman) because of Lucifer’s destiny to rebel and the eventual fight with his father and Michael, leading to the fall. War was the ‘force’ that was behind that fight (not sentient, but still there in principle) - technically, Lucifer brought ‘War’ into the world with him, though the ‘blame’ would be equally parsed between both him and his brother.
From there he tracked down through history, having an involvement in some way in every little feud and every global battle, starting with Cain and Abel, working his way slowly and steadily up to date with the Winchesters and the ‘potential’ reunion of the archangels to finish what they’d started.
Now he’s biding his time, waiting to see who the next frontrunner will be in bringing about the inevitable - the apocalypse will happen, at some point… Though right now it could be angels, demons, creatures or even mankind who sets light to the fuse and the horsemen will gather again…