Illinois has made several changes over the years.
http://www.ask.com/government-politics/did-illinois-change-legal-drinking-age-3dca6133c68e89a8
"After Prohibition ended in 1933, Illinois changed the drinking age to 21 for men and 18 for women. In 1961, Illinois raised the drinking age to 21 for everyone. In 1973, the drinking age for beer and wine was lowered to 19. In 1980, it was raised back to 21."
And here is one that talks about the idea behind the difference for gals and guys.
http://www.theatlantic.com/health/archive/2015/10/when-women-could-drink-at-18-and-men-at-21/409066/
My lifestyle was such that it really didn't make a noticeable difference but I got caught in the change from 18 to 21 for females.