If you want to look at Thomas Jefferson's census form to see how many slaves he owned, go to the National Archives, not the Census Bureau. The bureau must keep its raw data under wraps forever, but the archives gets copies and can make them public after 72 years. During the waiting period, the information is kept in a Pennsylvania coal mine. Afterward, the complete set is available at branch offices of the archives; some libraries keep complete or partial sets.
The public does have access to more recent data for some purposes. Individuals may obtain their own ``census transcripts,'' records relating to their own family, from the census storehouse in Pittsburg, Kan. The bureau used to get several hundred thousand such requests per year from individuals who wanted to establish eligibility for Social Security or prove they were related to rich Uncle Harry. So many Americans now have birth certificates that the number of requests for transcripts has fallen to about 40,000 a year.
Historians will find gaps. The census data for 1890 was destroyed in a fire in 1921, and other fires, including one started by the British when they invaded Washington during the War of 1812, wiped out parts of some years. All the records from the first census in 1790 are available.