Originally posted by Old Man Whirly!
it really hasn't DDM. He was 17 with a loaded gun, in a state where you shouldn't be 17 with a loaded gun. It just needs a witness or piece of video to show he had the gun raised at some point prior to the shootings and his case of any kind of defence collapses.
By the laws:
What you're referring to is it is illegal for a minor to possess a firearm:
Possession of a dangerous weapon by a person under 18....
(2) 
(a) Any person under 18 years of age who possesses or goes armed with a dangerous weapon is guilty of a Class A misdemeanor.
https://docs.legis.wisconsin.gov/statutes/statutes/948/60
However, another law allows for it in his particular circumstance:
Self-defense and defense of others.(1)  A person is privileged to threaten or intentionally use force against another for the purpose of preventing or terminating what the person reasonably believes to be an unlawful interference with his or her person by such other person. The actor may intentionally use only such force or threat thereof as the actor reasonably believes is necessary to prevent or terminate the interference. The actor may not intentionally use force which is intended or likely to cause death or great bodily harm unless the actor reasonably believes that such force is necessary to prevent imminent death or great bodily harm to himself or herself.
...
(1m) 
...
2. “Place of business" means a business that the actor owns or operates.
https://docs.legis.wisconsin.gov/statutes/statutes/939/iii/48
But persons under 18 can use the particular firearm that Kyle was holding under the exceptions section of 948.630.c. If you follow the exceptions statue, you end up on "29.304  Restrictions on hunting and use of firearms by persons under 16 years of age."
https://docs.legis.wisconsin.gov/statutes/statutes/29/iv/304
And Kyle's lawyer believes Kyle is cleared of the misdemeanor because of this.
This news article, who does not support open carry, also talks about this in less excruciating details:
https://www.chicagotribune.com/columns/eric-zorn/ct-column-rittenhouse-kenosha-self-defense-zorn-20200903-susvsg45yndn7pb67l42ywnzn4-story.html