It probably depends on the nation/state but here you only need a license to drive on public roads. The recommended age for a tractor is probably 14+ but I am willing to bet every farmers kid has driven one as soon as they were large enough to reach the pedals. Probably about 10-12, which is not too old for a house cat. Though probably rather old for an outdoor farm cat.
That's the point. The cats that have excess to tractors don't get that old and house cats aren't exposed to them. They will never develop the desire to drive a tractor.
I was 7 when parents first used me to start and stop the tractor driving along rows of fruit trees. Maybe less about reach than having enough weight to actuate the clutch pedal.