Three medieval source traditions give three different birth years for Genghis Khan (1155, 1162 and 1167), and no source contemporary with his birth survives to settle between them.
What would resolve this A securely dated Mongol or Chinese record from within Genghis Khan's own lifetime naming his age or birth year; none has been found.
OpenMedieval Mongol and Central Asian history; manuscript history of the Secret History of the MongolsThe Cambridge Medieval History