Yes. Its only illegal if you are under 18.
The hazards of riding in cargo areas have been addressed in 30 states and the District of Columbia
by a variety of laws, most of which are designed to protect children, but few of which
provide comprehensive protection for all children younger than 16. Seat belt and child
restraint laws also may apply to prevent people from riding unrestrained in cargo areas.
I see it all the time. That doesen't mean its legal.
I think in certain areas (legal or not) it is accepted as a way of life.