I expect they do get waivers for being LEO’s. But I don’t know what the age is for getting a gun is in Wisconsin..
From the NRA/ILA page for WI.
No state permit is required to possess a rifle, shotgun, or handgun.
It is unlawful for a person under the age of 18 to possess a firearm unless that person is accompanied by a parent or guardian and is target shooting or participating in a firearms training course. Young hunters between the ages of 12 and 16 may possess firearms under the supervision of a parent or guardian while in the field and/or while enrolled in hunter safety classes.
Purchase would covered by Federal Law. Essentially 18 for long guns, 21 for handguns.
But you are correct, these laws almost always have exceptions for police and military. Military are usually excepted only when possessing or carrying firearms in conjunction with their official duties. Police are generally excepted all the time.