I was 5. I remember casting on with the finger method and really wanting to learn the long-tail cast on. I also remember the elaborate motion of letting go of the needle to put the yarn over, so my tension must have been awful, but that's all I can remember. There are no finished masterpieces in the cedar chest from that period, so maybe that's as far as I got.
The funny thing is that I have no other knitting memories until after I was married and knitted a bulky sweater for my husband, but that wasn't hard, so I must have done some other knitting throughout my childhood, that I've forgotten. My mother did all the knitting for our kids and my mil loved to crochet, so there was no need for me to pick up the needles when I was a young, busy mom.
I didn't get serious about knitting until I retired at 50. Now I rarely sit down without picking up my needles and I always have at least 2 projects going. (I start my Christmas knitting for the next year before the decorations are down from the previous year.)