There is not such a statute, "that says that open carry is legal at 18." In principle, the law prohibits, not permits. If it is not explicitly illegal then it is legal.
Would you look for a statute that allows attending church on Thursday?
948.60 Possession of a dangerous weapon by a person under 18.
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(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.