I shall skew your poll a little ;) I started writing fanfiction at age 4 1/2, when requested to write my first compositions in primary school. They featured Bilbo, usually doing something OOC with flowers. I had no knowledge of fandom or copyright. I branched into original fiction aged 7 or 8, with a carload of people with some bodies on the roofrack, and blood running down the rear windscreen. There was also a burial, which word I was completely unable to spell.
There were offically sanctioned forays into fanfic at school, where one was meant to write 'missing scenes' for things like Shakespeare, Sebastian Faulkes, etc. Again, I didn't know about fandom. ;)