Not only does a kid's maturity, physical strength, and intellect/wisdom matter, but the people and working environment around them matter a lot too.
The summer I was 17, I worked on a ranch out on the dessert. The rancher had a 7-year-old kid and a 14-year-old nephew that both helped me out. The nephew was some city-slicker kid that was sent to his uncle's farm to "straighten him out".
I'd trust the 7-year-old kid better than the 14-year-old nephew on any of the tractors with any of the implements that we used, but never when the nephew was around. The nephew had a way of coming up with "bright ideas", and he was a bad influence on the kid as far as wisdom and maturity went.
The boss told me to give the kid as much experience as I could when the nephew wasn't around. In fact, the boss's wife insisted that the kid drive any time we were out working. In her mind that was the safest place for the kid to be.
That brings up the other point I'd make...It's important that the kid is ready, but when is the mother ready?