There were several reasons why our population was young and growing in the early 1800s. One reason was that immigration to our country increased, especially from North and West Europe, in the early 1800s. Coming to the United States was a difficult task. It was more likely that younger people would come to the United States. Once they got settled they would get married and raise families. In some cases, young families came with young children. They might have more children after they got settled. As their kids got older, they got married and raised families.
Another factor was that many Americans were farmers. The farmers often had large families because they needed a lot of work to be done on the farm. The children could help with the various jobs that needed to be done. Their kids often got married at a young age and began to have families themselves.
There were some other factors involved. The birth rate exceeded the death rate, leading to an increase in our population. As technology and innovation developed, it also became easier to deal with diseases, lack of food, and other issues that could cause people to die at a young age. For example, new machines made it easier to grow and harvest more crops. This helped to provide more food for the American people.