I came from exactly that sort of community. The fact of the matter is that teen would have driven that truck regardless of the law permitting it.

IMO, this sort of thing should work more like the way fair use works. A cop could pull you over for a traffic violation, ticket you, and then when you go to court you push the defense of "I'm a farmer and I was doing farm work" to get the missing license charge dropped (but you'll still likely end up with a traffic ticket to pay).

Generally speaking, cops aren't patrolling farming roads anyways so you'd really not need almost any exemption in place.