Personally identifiable information collected for the decennial census is not released by the government for 72 years after it was collected. After 72 years, the records are released to the public by the National Archives and Records Administration. [source] In 2012, individual-level records from the 1940 census were released; 1950 census records are due to be released in 2022.
For scholarly research purposes, this means that 1940 and earlier census records can be used to search for information about individuals. For example, a genealogist can search for records by name and see the names, ages, occupations, and other information about specific individuals. For 1950 and later decennial census records, researchers must use aggregate data (with the exception of Public Use Microdata, a special kind of anonymized individual-level data set.)
After 72 years, census records become the domain of the National Archives, which makes census schedules all the way back to the first decennial census in 1790 available on microfilm, and increasingly as digital images online. [source]
https://www.census.gov/programs-surveys/acs/guidance/training-presentations/acs-intro-pums.html
https://usa.ipums.org/usa/
https://www.census.gov/about/policies/privacy/statistical_safeguards/disclosure-avoidance-2020-census.html
https://www.aeaweb.org/articles?id=10.1257/pandp.20191107
https://www.thetaskforce.org/queerthecensus.html
https://www.census.gov/newsroom/press-kits/2020/2020-census-lgbtq.html