In Johnny Tremain, Johnny plays an important role helping the rebels in the American Revolution, despite his young age. Although this would be highly unusual today, it is historically realistic. The lives of children and teenagers were very much affected by the Revolutionary War, and many of them played active roles in the conflict.
On both sides of the war, soldiers would often bring their wives and children with them on campaigns. Although they did not go to school, these young people were still expected to work. They would do chores around the camp like gathering water (as Cilla does in Boston) and delivering messages (like Johnny). If a boy of Johnny's age had musical talent, he could be recruited as a musician to play the drums or the fife for the regiment. This was a paid job. However, many of the younger children had to do unskilled work that was only paid in food rations.
According to the Van Cortlandt house website, even if a child's father wasn't fighting in the war, their family could still be displaced. Many rebels in Tory-controlled territory feared for the safety of their families, and relocated to parts of the country that were more sympathetic to rebels. Tory families did the same thing when they found themselves in rebel-controlled towns and colonies. While they were on the road, children and teenagers had to do many of the same chores they would have done in a military camp.
If they weren't displaced, teenagers could and did participate in the fighting, as described by Metz. In colonial America, more than half of the population was under 16. Starting at age 16, boys could be drafted by the British or by the rebels. However, many volunteered to serve when they were much younger than that. Since boys were taken from home to fight at a young age, their sisters usually had to pick up the slack in household chores, taking on a bigger workload and sometimes helping out with the family business. Like Johnny, many teenagers worked as spies, passing information to Tory or rebel leaders.