According to the Insurance Institute for Highway Safety, state laws permit individuals age 16 and older to ride in the bed of a truck without any kind of restraints or supervision. The state also allows individuals 15 and under to ride in the bed of a truck with a couple of provisions: an adult must accompany the minor, and some sort of seat belt is required unless there is a permanent overhead structure in place.
States constantly revise truck bed laws to help protect passengers. Truck beds are designed to carry cargo, not passengers, and it's easy for passengers to fly out from the back even at slower speeds. Sharp turns, bumps in the road, and other hazardous driving conditions can throw passengers around or cause them to fall out. Since there are no restraints in the back of a truck, most states have age restrictions for truck bed passengers.