The American FactFinder webpage through the Census Bureau allows you to find statistics based on many factors, including age, sex, region, employment, etc. This site also will allow you to perform complex searches, such as finding the number of people by age, income and sex who live in the Northeast urban region of the country, a state or city.
Here are two short videos that show you how to navigate this site:
General population statistics:
Regional (West, Northeast, urban, rural, etc.) information: