Answer:
From the Illinois Constitution:
To be eligible to serve as a member of the General
Assembly, a person must be a United States citizen, at least
21 years old, and for the two years preceding his election or
appointment a resident of the district which he is to
represent. In the general election following a redistricting,
a candidate for the General Assembly may be elected from any
district which contains a part of the district in which he
resided at the time of the redistricting and reelected if a
resident of the new district he represents for 18 months
prior to reelection.