I helped yank a motor from a wrecked stock car when I was 3 (I was running the crane while dad and his buddy had their fingers in there). I got to play with the drill press, grinder, and welder before I was in school.
No, I didnt grow up on a farm. But dad DID, he was overhauling tractor motors at 10, and driving at an obscenely young age (first ticket at 12: no license).
I knew how to drive a car before I was in high school, and was playing with carbon composites at 15 (left over b2 bits, whee).
It IS a knack for these sorts of things, but also having someone around to help get you started, combined with an interest in what ever you're working on.
This is no different than how most of you learned to program. Well, I guess it is: you're not sitting on you butt while you do it :)