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 https://law.justia.com/codes/wisconsin/2018/chapter-941/section-941.231/But to be on the safe side it may be best to leave the knife at home to avoid any potential issues.
Wisconsin laws are a bit of a confused mess when it comes to their weapons laws.
Under the State Fair Park Board's section 2.14, however, the "dangerous weapons" definition includes knives: https://docs.legis.wisconsin.gov/code/admin_code/sfp/2/14
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.