User blog comment:SayuriDarling/The House of Hades Official Discussion/@comment-24308873-20131020014711/@comment-3146930-20131020220115

Out of the three brothers, Hades was the oldest. Zeus knew after the war with the Titans that by birthright, Hades would want to become the king of the gods. So Zeus decided on playing a game of stick to decide who would rule what. Hades got the Underworld, Poseidon the Oceans, and Zeus got the sky, becaming king in the process. If you're going by outright age however, Hestia would technically be the oldest.