Aiden4's answer about Winconsin's statute 948.60 is correct, but incomplete and the complete reason is interesting/funny, so I'll expand on it:
The statute reads:
948.60 Possession of a dangerous weapon by a person under 18.
(1) In this section, “dangerous weapon" means any firearm, loaded or unloaded; any electric weapon, as defined in s. 941.295 (1c) (a); metallic knuckles or knuckles of any substance which could be put to the same use with the same or similar effect as metallic knuckles; a nunchaku or any similar weapon consisting of 2 sticks of wood, plastic or metal connected at one end by a length of rope, chain, wire or leather; a cestus or similar material weighted with metal or other substance and worn on the hand; a shuriken or any similar pointed star-like object intended to injure a person when thrown; or a manrikigusari or similar length of chain having weighted ends.
(a) Any person under 18 years of age who possesses or goes armed with a dangerous weapon is guilty of a Class A misdemeanor.
(b) Except as provided in par. (c), any person who intentionally sells, loans or gives a dangerous weapon to a person under 18 years of age is guilty of a Class I felony.
(c) Whoever violates par. (b) is guilty of a Class H felony if the person under 18 years of age under par. (b) discharges the firearm and the discharge causes death to himself, herself or another.
(d) A person under 17 years of age who has violated this subsection is subject to the provisions of ch. 938 unless jurisdiction is waived under s. 938.18 or the person is subject to the jurisdiction of a court of criminal jurisdiction under s. 938.183.
(a) This section does not apply to a person under 18 years of age who possesses or is armed with a dangerous weapon when the dangerous weapon is being used in target practice under the supervision of an adult or in a course of instruction in the traditional and proper use of the dangerous weapon under the supervision of an adult. This section does not apply to an adult who transfers a dangerous weapon to a person under 18 years of age for use only in target practice under the adult's supervision or in a course of instruction in the traditional and proper use of the dangerous weapon under the adult's supervision.
(b) This section does not apply to a person under 18 years of age who is a member of the armed forces or national guard and who possesses or is armed with a dangerous weapon in the line of duty. This section does not apply to an adult who is a member of the armed forces or national guard and who transfers a dangerous weapon to a person under 18 years of age in the line of duty.
(c) This section applies only to a person under 18 years of age who possesses or is armed with a rifle or a shotgun if the person is in violation of s. 941.28 or is not in compliance with ss. 29.304 and 29.593. This section applies only to an adult who transfers a firearm to a person under 18 years of age if the person under 18 years of age is not in compliance with ss. 29.304 and 29.593 or to an adult who is in violation of s. 941.28.
2 things to note:
- (1) takes care to include, in the list of dangerous weapons: nunchaku, shuriken and manrikigusari. While the first 2 are more or less familiar to everyone knows anything about Japanese martial arts, the last one had to be looked up by everyone following the case to discover that it's the "secret weapon of the Ninja"(even more than the shuriken).
- (3.c) says that the whole of this entire section applies[adding the brackets to make following the formal logic easier] only if (the person under 18 is in violation of 941.28[barrel length under 16 inches]) or (is not in compliance with ss. 29.304[Restrictions on hunting and use of firearms by persons under 16 years of age] and 29.593[Requirement for certificate of accomplishment to obtain hunting approval]).
In programming terms(for those so inclined), 3.C could be written as:
IF ((barrelLengthInches < 16) OR (huntingUnder16Applies AND huntingCertificateApplies))
Since the barrel length is over 16'' and Rittenhouse is over 16 and no hunting permit was required for his activities, the whole section of the law did not apply. Assistant District Attorney James Kraus argued that the exception renders the state’s prohibition on minors possessing dangerous weapons meaningless. In essence, that the legislators drafting that law spent too much time watching cheesy early 90's action movies and thinking of how to save Wisconsinites from the Ninja threat, to draft the law properly, so it should be read according to its intent from the title of the section.
However, there is a binding Common Law precedent, dating back from the 16th century called the "Rule of Lenity", also called "Strict Constructionism" in the US, whereby if the legislature screws up, it's the legislature's problem. In the original case, the law in England forbade "felonious stealing of Horses, Geldings or Mares". A thief was caught, but argued that since he only stole one horse and the law specified horses, the law didn't apply to him. He was let off and the law hastily rectified. Pre-revolutionary Common Law precedent is binding in the US and it was re-affirmed multiple times, e.g. United States v. Wiltberger, where a US sailor got off with killing another US sailor in a Chinese estuary, because the law only applied on the "high seas". So, the charge was tossed and the defense didn't press the issue further.
However, the really interesting bit is that even though it didn't get to be argued since Rittenhouse was 17, the way the law is actually written, this section only applies if (huntingUnder16Applies AND huntingCertificateApplies). That means that there is literally nothing in Wisconsin barring a 12 year old(under 12 is separately forbidden in the 29.304/huntingUnder16Applies section) from possessing and using an AR-15(or AK-47), as long as the barrel is >16'' and a hunting license isn't required for the activity.
I think that the legislature will amend the law with haste, before it can be tested on 12 year olds.
P.S. the other guy who gave him the gun will get off with this precedent too, since the statute for his charge is:
This section applies only to an adult who transfers a firearm to a person under 18 years of age if the person under 18 years of age is not in compliance with ss. 29.304 and 29.593 or to an adult who is in violation of s. 941.28.
i.e. the same 3 sub-sections as for Rittenhouse.