User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5774152-20121226062547/@comment-3146930-20121226065634

The first Great Prophecy said that the next child of Poseidon, Hades, and Zeus (not their Roman forms) would make a choice that would save or destroy Olympus. The first child of the Big Three to reach 16 after the Great Prophecy had been spoken was Percy. After the prophecy was completed, then they move on to the next Great Prophecy which was the Prophecy of Seven.

The Prophecy of Seven however does not have any kind of age limit or anything.