The Constitution of Wisconsin is clear, but makes no mention of age
"Right to keep and bear arms. SECTION 25. [As created Nov. 1998] The people have the right to keep and bear arms for security, defense, hunting, recreation or any other lawful purpose."
And neither does the Wisconsin Statutes
If you plan on carrying a knife, concealed or otherwise, check with the local police first. There are laws that apply to possession of a dangerous weapon by a person under the age of 18, but the "where" and "why" of carry can make a difference.