I've only found 3 in my searches for arrowheads. One of those was a very old clovis point. According to a pro and a few ID books I've looked at it could very well date back to the ice age....which is pretty damn old.
Considering that I live in a very interesting geological area, it's very possible that it could be that old. Unlike most of Northern Illinois, glaciers didn't completely flatten the landscape, leaving us the Southern Illinois Ozarks.