Final answer:
The legality of transporting children in the bed of a truck without a shell varies by state, with no universal age restriction. To determine the correct age, one must consult the specific vehicle and traffic laws of the state in question.
Step-by-step explanation:
The legality of children riding in the bed of a truck without a shell varies by state in the United States, and there is no specific age that is universally applied across all states. Most regulations concerning this issue are set at the state level, not the federal level. The information provided from the historical context about the federal law passed in 1938 relates to child labor and is not directly applicable to laws governing the transportation of children in vehicles. To find the correct age restriction for riding in the bed of a truck, it is necessary to refer to the vehicle and traffic laws of the particular state in question, as these laws are not standardized federally.