Minimum age for possession in WI is 18. In IL he'd need an FOID, which requires parental permission if under 21, to possess or purchase. Presuming the rifle was his parents, they've got some liability. An unfortunate situation.
Strange that Wi has no minimum age for handgun possession.
It's entirely possible that the shootings were justified via self-defense. That will come out in a trial.
However as you note, he was in possession of a firearm and was under the age of possession in WI. *If* he crossed state lines with a firearm, that comes under federal jurisdiction - and more potential problems.
And yes, where were the parents?
With that said, I'm not sympathetic for rioters that screech this was an outage when people have been seriously injured and property destroyed and in many cases local law enforcement has done little.