You guys are right.
Some states don’t even have minimum age requirements for possession of a rifle. There is no federal minimum.
In Wisconsin it is 18.
In Illinois it is 21.
So if he didn’t steal it, somebody committed a felony transferring it to him.
Not necessarily. Allowing someone to use a firearm is not the same as transferring it to them in most states.
Here is what the government of Wisconsin says about the topic:
Persons under age 18 may not possess firearms for non-hunting purposes unless accompanied by an adult, except that persons ages 14–17 who have completed Hunter Education can possess legal shotguns and rifles without being accompanied by an adult.
If one of the other people defending the car dealership was an adult, then nothing about Rittenhouse's possession of a rifle was illegal. If Rittenhouse took the Wisconsin hunter education course the same thing is true.