Midnight God takes place 5 years after Conan married Zenobia, and their first child was murdered by Ra-Sidh, so I'm guessing that at this stage of the game Conan's late 60s. Conan would have been an old man when he sired children with Zenobia.
Though I'm wondering what his "final adventure" will be.
He's faced the first evil god ever (triumphing only because the wounds the god had, a convienent flashback of dear old granpapa and the giant kings of acheron giving him advice on where to attack), the most evil acheronian sorceror ever (and he still needed Hadrathus's help to triumph), numerous evil sorcerors (thugra khotan, the master of yimsha, and if we're going by dark horse continuity that avatar that thoth summoned)
Well technically speaking it's the other way round: Hadrathus defeated Xaltotun with the help of Conan, Zelata and her wolf. Conan's fight was with Tarascus, and we know how that turned out.