315 ILCS 20/3-12

"Conservation Area" shall mean an area in which the structures in fifty per cent or more of the area are residential having an average age of thirty-five years or more. Such an area by reason of dilapidation, obsolescence, or deterioration, or illegal use of individual structures, overcrowding of structures and community facilities, conversion of residential units into non-residential use, deleterious land use or layout or any combination of these factors may become a Slum and Blighted Area.