A U.S. passport is valid for 10 years if you were 16 or older when it was issued, and for 5 years if you were under 16, according to the U.S. Department of State. The wallet-sized U.S. passport card has the same validity periods as the passport book (State Department, passport card vs. book). You can find the issue date on the data page of your passport book or on the front of your passport card.
A passport issued to a child under 16 cannot be renewed; a new application must be made in person. Also check the expiration date before booking a trip abroad: some destinations require a passport to be valid at least 6 months beyond the dates of travel, and some airlines will not let you board if that requirement is not met.
Source: U.S. Department of State, “After You Get Your New Passport”.
Image: Older and newer U.S. passport book covers, U.S. Department of State, public domain, via Wikimedia Commons.