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.Source
If one of the other people who spent the evening with Rittenhouse 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.
I am glad to know that.
One issue down, one to go!
I read or heard somewhere that Rittenhouse lives in another state and traveled there...?
It would look better if he was there with his father, but it sounds like he was just there on his own accord.
The Twitter rant is he was underage and brought an Illegal Firearm across State Lines.