Demographics are a subset of statistics that focus on people. In the United States the Census Bureau is mandated to keep track a lot of this information.
It is also important to consider how often data is collected. In the United States the Census is taken every ten years, with the next collection being in 2020. You may be able to find research with more recent demographic information, but it will not be on the scale of the Census.
Tip! Census questions have changed over time, keep this in mind when looking at historical data.
If you are looking for International demographics, World Bank has a lot of information that can be helpful. Many other nations also have some sort of census collection and you can try navigating their governmental websites for this, but this can be much more difficult.