There are several things that will disqualify you from owning a firearm in IL, such as: Not being a US citizen or legal alien. Having a medical marijuana license (this prevents you from purchasing from an FFL, but does not prevent you from owning) Being convicted of a felony.
You may be found guilty of unlawful possession of a firearm if: You are under 18 and possess any firearm of a size that may be concealed. You are under 21, have been convicted of a misdemeanor other than a traffic offense or found to be a delinquent and have any firearms or ammunition in your possession.
If you are caught without registration, it is at minimum an automatic Class A misdemeanor.