As was the case for many of us here, I read a lot, and was making my way through rather hefty adult novels by the time I was 7-8 (although I was much more into factual books than fiction, it must be said). I was also the designated "smart kid" throughout schooling, so I suppose it's hard not to fall into the nerd category when that's the case.
Earliest evidence? When I was 3-4, I became very interested in snooker - simply because the mathematics involved (with each colour ball being worth a certain amount of points) fascinated me. I loved watching matches and calculating scores. Combined with the fact that I could tell the time when I was 2, I suppose that early interest in mathematics was a sign of Nerdian heritage, although ironically maths would become my least favourite subject in later years.
When I was 5, I entered my dinosaur phase, which hasn't yet ended, and I pretty much devoured any and all books on prehistoric life. I would draw dinosaurs whenever I could, and I eventually wrote a dinosaur-based "choose-your-own-adventure" book, which if I may say so was quite impressive for my age (I put real effort into it). In the later years of primary school I moved onto science fiction and aliens (I recall drawing Hork-Bajir having seizures because one of the trees they feed on is the fit-fit
. This was amusing to me).
I was always amazing my teachers by, well, being nerdy. The painting I created when I was 8 impressed my teacher so much she had my mother called in to gush over it (my mother still has it framed on the wall). When I started nursery at 4 I pointed out to the head-teacher that there were "three too many" candles on the cake celebrating the fourth birthdays of several of us children, which apparently really surprised her. Then there was my, er, "passion" for acting, which we know as the Sheep Incident.
Most notably, I never played sports at school, which is surely one of the definitive symptoms of youthful nerditude, particularly for boys.