US Census records from 1930 and earlier have been made public, and are available at sites like ancestry.com, and familysearch.org. For more recent Censuses though -- from 1940 through 2000 -- the individual records are still held as confidential. But that doesn't mean you can't get at them! Here's how.
Check Out the Age Search Service at the Census Bureau
The Census Bureau is authorized to release certain census records for individuals in order to provide legal confirmation of information such as age, place of birth, and so on.
The information can be released for official reasons, such as passport applications or for social security. But it can also be released for other reasons, including genealogical (family history) research.
Head to census.gov/genealogy and click on the menu item for "Age Search" for full details.
Check requirements carefully. In general, such census records are limited to close family, or those with a legal relationship to the individual in question, such as a guardian or executor of an estate.
Submit an Application
If you are eligible to receive a census record, then submit the Application Form (Form BC-600) available at the Census Bureau website, along with the required fee ($65, at the time of this writing).
Census records from 1930 and earlier are available at some commercial websites. The Resources section includes links to some of the best family history sites.