My kids started summers on lake in Ontario when they were 3 and newborn. I was just starting fly fishing at the same time. They had great fun with a closed-face spinning reel and worms (that I put on the hook). My youngest would actually have as much fun sitting in the bottom of the canoe playing with the worms as he did fishing. Fishing for even easily caught perch, the best game/challenge was, "Can you catch 4 fish when you are 4?", "Can you catch 5 fish when you are 5?". That about matched their attention span and also marked - for both of us - a time limit. Yes I heard, "Can I try that?" - but that's what they wanted -to try it, not do it. I agree that maybe about 10 it the right age because by that time they will not only have the muscles and understanding, but they will also be old enough to be left alone on the dock or river without your constant attention.
Good luck with it though, there is nothing better than fishing with your kid. My 4 year-old grandson did catch a perch this summer on spinning gear at the lake. He had a great time retrieving the lures, I did the casting.