Actually about a month before I turned 5. Miniapolis-Molein came out with what they marketed as a "Childs" tractor. It was smaller than regular, but still could do farm work. It fit the "kids" so as they could work too. It was NOT a toy. I learned to drive it and became the kid who could rake hay. From there I just kept driving bigger equipment including cars / pickup by age 6 and grain trucks by age 8, combine by age 10.
My friend Calvin, and I overhauled a International Super "C" (it's a smaller tractor, but full size) with out any adult help. I was 7 and he was 9, he was the brains but I helped quite a bit.
Oh those were the days, growing up on the farm in the 60's.
Salute the nation