Our Dad always let us steer the boat since I could ever remember, but when I got to take out a boat one my own I was 14...grew up on the lake, was always in a boat, but had to know how to swim first...the swimming part was a no brainer. He knew when we (5 of us) were ready...had to know the rules of boating..I can't tell you how grateful I am that it all happened naturally...I've never forgotten the where, when's and how's! Still boating up the "big lake", after all these years!
Be stern, and kind, and make sure your kid knows when he messed up, and they'll do great!