I was about 8 when I first started to learn to drive the tractors on our farm. By 12 I was driving everything on the place. The smallest tractor, our 1939 Allis Chalmers WC, was also the hardest to drive because of the long, heavy clutch, the lack of power steering, and the individual hand brakes by the fenders so I actually drove some of the bigger tractors first.
As for my kids, my oldest is a natural and I let him drive our tractors by age 12 when we first got our place. He was already racing motocross by then and had a pretty good understanding of how machinery worked. My stepson who is in between the ages of my oldest and youngest, I still don't really trust to drive the tractor, and he's 21. He just never had the attention span to learn and was not naturally so inclined. My youngest is kind of in the middle. He can operate it fine, but really did not feel comfortable until he was about 16. My oldest grandson is 20 months old, and loves to ride on the tractor, and wants to see what everything does. We'll just have to see whith him.