Ordinarily, a US passport is valid for 10 years for adults and five years for children under age 16. Your passport’s expiration date isn’t the only factor to consider when traveling internationally. Many countries require your passport to have a specific period of validity remaining. Since you cannot travel on an expired passport and it takes time to get a new passport, countries try to ensure visitors won't have any trouble getting back home should their planned departure be delayed. Many have adopted the six-month passport rule to avoid complications.

Source: CIBTvisas