Illinois became a part of the Northwest Territories in 1787. It became the Illinois Territory in 1809, being taken out of the Indiana Territory. It became the Twenty first State in 1818. The State then opened up and became populated in the 1820s.
The first Federal Census for Illinois was in 1820. State Censuses were taken in the odd years of 1825, 1835, 1845, 1855 and 1865. These are included among the Federal pages on the link to 1820-1865.