U.S. citizens of all ages traveling between the United States and other countries by air need to present a valid U.S. passport.
All American citizens now need either a passport (for all modes of travel and for anywhere in the world) or a passport card (for land or sea travel only between the United States and Canada, Mexico, Bermuda, and the Caribbean).
Note that parents traveling with small children should bring photocopies of their children's birth certificates to avoid any problems.
U.S. Department of State (877/487–2778. travel.state.gov.)