The short answer is this: If you mean full-time work (with certain exceptions) ―none were employed, at least legally.

This was the result of the signing into law in 1938 (two years before) of the Fair Labor Standards Act. Wake Forest University Economics Professor Robert Whaples puts it succinctly: "the Fair Labor Standards Act of 1938 prohibited the full-time employment of those 16 and under (with a few exemptions) and enacted a national minimum wage which made employing most children uneconomical." Whaples describes the various reasons, apart from the enactment of federal, state, and local laws, why the numbers of children working in industry and on the farm had already declined dramatically over the first few decades of the 20th century before the passage of the Act.

The provision of the Fair Labor Standards Act (FLSA) that dealt with child labor stated: "No employer shall employ any oppressive child labor in commerce or in the production of goods for commerce or in any enterprise engaged in commerce or in the production of goods for commerce."

The FLSA, as further defined by statute, prohibited children under 16 from any kind of employment in certain hazardous kinds of work. The Secretary of Labor has periodically revised such categories of hazardous work. For agricultural work, for example, those under 16 should not be employed in operating a tractor of over 20 horsepower, or a hay baler, a forklift, a power post-hold digger, or a chain saw. Nor should they work in a yard or stall in which there is a bull, boar, or stud horse, a sow with suckling pigs, or a cow with newborn calf.

The FLSA also prohibits those under 18 from certain kinds of occupations altogether. These include occupations that require working with explosives or radioactive materials, operating most power-driven woodworking, baking, or meat processing machines, as well as most jobs in mining, meatpacking, logging, and brick-making.

The FLSA exempts certain kinds of work, including employment of children by their parents, and church work. Boys and girls also worked (as they do today) at jobs not covered by FLSA regulations, such as office or clerical work, retail sales positions, food preparation, caring for younger children, and so on.

Given these exemptions, what was the actual number working in 1940?

Gertrude Folks Zimand, General Secretary of the National Child Labor Committee, in "The Changing Picture of Child Labor," published in 1944 in the Journal of the American Academy of Political and Social Science (pages 83-91), reported that the 1940 census showed, among 14- to 15-year-olds, a total of 4,347,665 attending school, and a total of 209,347 gainfully employed (no number was reported for children ages 7-14 who were not in school but were gainfully employed, but the number of these children attending school was 15,034,695; the numbers for 16- to 17-year-olds were 3,361,206 in school and 662,967 gainfully employed). The term "Gainfully employed" included full- and part-time work, either in industry or agriculture. There was some overlap in these numbers because some children were in school but were also working, at least part-time. But according to Zimand, 64 percent of the 14- to 15-year-olds and 83 percent of the 16- to 17-year-olds who were working were out of school and were therefore presumably working full time.

In colonial America, there were not really any “teenagers” as we know them. Of course, children passed through the decade that we know as the “teens,” but that stage of their lives was not the carefree, exploratory period that today’s youth experience. Children grew into adulthood more quickly than they do today, and by the time a child entered their teen years, they were already on a path toward their life’s occupation. Although a youth’s path to adulthood depended on their family’s socio-economic status, regardless of wealth, young men usually learned their trade through some form of apprenticeship.

Children from poor families were often bound out to servitude at a young age, earning their keep while learning a trade. In the seventeenth century, 80% of the Chesapeake’s immigrants were indentured servants. Many of these servants were over the age of 20, but a significant number were young men and women still in their teens. In return for their passage to the Chesapeake, these servants agreed to work for a period of time, usually between four and seven years, without pay. During their service, masters provided food, clothing and shelter, and at the end of their term, servants received “freedom dues,” usually three barrels of corn and a suit of clothes. Many of these youths were orphans, but some were from indigent families who could not care for their children, and therefore sent them off to find their own fortunes. But a young person did not have to travel across the Atlantic to enter into servitude. At the age of 12, Benjamin Franklin was apprenticed to his brother, James, a Philadelphia printer. In his autobiography, Franklin recalled that his brother was “passionate” and often beat him. “Thinking my apprenticeship very tedious,” Franklin stated, he “was continually wishing for some opportunity of shortening it.” At the age of 17, Franklin ran away from his brother’s household.

Children of wealthier families also took on considerable responsibility at a young age. Children from families of middling means often learned how to read and write, especially if they lived in urban areas. By the time they were in their mid-teens, sons were at work in the family farm or business, learning the trade that they would probably practice the rest of their lives. In the wealthiest families adolescent boys were often sent to boarding school, and then when they were around 15 years of age, they entered institutions such as Harvard, William and Mary, or Yale. After finishing their formal education, many took apprenticeships as clerks in merchant offices or law offices, or they returned home to follow their fathers’ profession.

Only young men were allowed to pursue higher education. Although there were a few opportunities for girls to receive a more extensive formal education in the colonial period, most families kept their daughters at home to learn how to run a household and to be a dutiful mate for her future husband. There were rare exceptions to this convention, however. In 1738, when Eliza Lucas was 15, her father moved her and her mother from Antigua, West Indies, to a plantation near Charleston, South Carolina. Her father’s travels with the army and her mother’s ill health forced Eliza to manage the family business. In a letter to a woman friend, Eliza Lucas described her duties: “I have the business of 3 plantations to transact, which requires much writing and more business and fatigue of other sorts than you can imagine.” But, she assured her friend that “I think myself happy that I can be useful to so good a father, and by rising very early I find I can go through much business.” Lucas assumed an unusual burden as a young woman, but eventually she followed a somewhat conventional path – she married a planter and had children, yet she continued to hold considerable responsibility in the management of her husband’s plantations.