What are the qualifications to be an Illinois state senator or state representative?

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.