From the Guide to 2010 State and Local Census Geography – Indiana – History:

The area of Indiana was part of the original territory of the United States, being part of lands ceded by four states to the United States and designated in 1787 as the "Territory northwest of the River Ohio." Indiana Territory was organized on May 7, 1800, from the western part of the Northwest Territory; it included all of present-day Illinois, nearly all of Indiana and Wisconsin, the western part of the Upper Peninsula of Michigan, and northeastern Minnesota. Indiana Territory was greatly reduced by the organization of Michigan Territory in 1805 and Illinois Territory in 1809. Indiana Territory was increased in 1816 by the addition of a strip of land that set the northern boundary between Indiana and Michigan territories and was reduced by relinquishing territory in the Upper Peninsula to Michigan Territory. Indiana was admitted to the Union on December 11, 1816, as the 19th state, with generally the same boundary as the present state.

Although the territory had not yet been legally established, census data for Indiana are available beginning with the 1800 census. The 1810 population includes a small population in territory now within Michigan (most of this area was not enumerated, being primarily unceded American Indian lands). For an explanation of the revision to the 1800 population of Indiana, see Richard L. Forstall, Population of States and Counties of the United States: 1790-1990, Washington, DC: U.S. Government Printing Office, 1996, page 52.

Data for the legally established state of Indiana are available beginning with the 1820 census.