I tried to teach a boyfriend's 5-year-old daughter to knit. No go - she was simply too young and too uncoordinated. So I gathered Jenny into my lap, put her hands on my needles then my hands atop hers.. and we knitted together, with lots of love and grins.
It's vital that children associate knitting/crocheting with love and good caring warm-fuzzies.
Then the knitting/crocheting will come when the child is ready.
PS - I have taught 8-year-olds, about 5-6 at a time.