...semi-aside, a young kid driving in a driveway would be perfectly legal, right? Private property and all that?

I remember meeting farm kids who were offspring of one of my mom's college classmates who routinely drove around the farm back in the 70s.

Still normal as soon as they can reach the pedals. If they're on a farm they're probably also riding on (or driving) small tractors and four-wheelers, and maybe riding horses, all of which are a hell of a lot more dangerous than towing a trailer of square bales a quarter mile over a private dirt road in the old '94 Ford, from the field to the barn, probably not going over 15mph because you'll rattle your bones out of your skin and the windows out of their frames if you go faster. Hell driving on a farm is probably safer than wandering around the more developed parts playing. There are lots of ways to get hurt on the kinds of structures, equipment, and junk in the more developed areas of a farm, and there's the livestock, if there is any, which can also be very dangerous.

This is still routine today. Most of the farm kids I know were driving early, some before 10.

That's how I first learned to drive, doing laps around the front pasture in my mom's S-10.

IANAL but I believe so. When I was very young, long before a license, but probably closer to 10 than 5, my parents taught me to drive the tractor around the field. I think others here will have had similar experiences as well.