Greetings Tim:
Please visit the Website of the Illinois Big Tree Register: http://web.extension.uiuc.edu/forestry/il_big_tree.html
Information about all our native champion trees is listed on this site.
To age this tree, you essentially have three options, two of which are logical: cut the tree down and count the rings (not practical or condoned); use an increment borer to extract a core whereby you can count the tree rings; or find out when the tree was planted (historical records for urban plantings, not so feasible for forest-grown specimens).