The Age to Lawfully Own and Possess a HandGun is 18 Years of Age, under Federal Law, however; Federal Law 18 U.S.C. 922(x) Prohibits the Purchase, Sell, or Transfer of a HandGun to anyone under 21 Years of Age.
In most States, one has to be 18 to Own a Handgun, and 18 to Apply for, and Recieve, a HandGun Permit.
Under Wisconsin Code 948.60(2)(a), The Age to Own AND Possess a Firearm is 18 Years Old, as it is in MUCH of The United States of America.
Since Open Carry is 'De Facto' Lawful, at 18 Years of Age, Open Carry of a Loaded Handgun is Legal in Wisconsin.