The United States in the 1700's was predominately comprised of young people for a variety of reasons. Life expectancies were much shorter than they are today, primarily because so many people died in infancy or childhood. That meant that a relatively small percentage of the population lived to old age.
Other that the Native Americans, the country was populated almost entirely by immigrants, many of whom came came from England or other parts of Europe. Getting here required an extremely hazardous ocean voyage, and life on the new continent was equally difficult. The immigrants who came here voluntarily were either fleeing from persecution or were poor and hoping for a better life in the new world, the only reasons they would take such a risk. Few people who were old or ill would have attempted such a trip, and many of those who did probably died in the process. The slave population was also young. Slave-traders were not going to waste their time bringing over those who were old or ill, and relatively few slaves survived long enough in the harsh conditions most of them endured to live to old age.