948.60(2)(a)(a) Any person under 18 years of age who possesses or goes armed with a dangerous weapon is guilty of a Class A misdemeanor.

948.60(3)(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.

29.304(1)(b)(b) Restrictions on possession or control of a firearm. No person under 12 years of age may have in his or her possession or control any firearm unless he or she is enrolled in the course of instruction under the hunter education program and he or she is carrying the firearm in a case and unloaded to or from that class under the supervision of his or her parent or guardian, or by a person at least 18 years of age who is designated by the parent or guardian, or is handling or operating the firearm during that class under the supervision of an instructor.

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Rittenhouse is under 18, so would be guilty of a class A misdemeanour, but only if he's in violation of 29.304. And 29.304 is the hunting statute, with certain requirements related to hunting, which I summarised as "for hunting."