What is the case if a minor commits a crime and then prosecuted when he/she is over 21 under illinois law? (just curious, for law class)?

What happens depends on BOTH the jurisdiction, AND the crime. In some cases, they couldn't be prosecuted after 4 years, even if they were a legal adult at the time of the crime. In others, they would still be tried and sentenced as if they were a juvenile, and they could be held by the juvenile system up to age 25. In still others, they could have been tried as an adult at 14, and will still be tried as such.

They charge him like a child and then put him in adult prison. Regardless he still gets repeatedly raped by brawny, black men while the jailers play cards mere meters away. Then they'll put lipstick made of catchup on you, and name you Suzy.