My mom likes the tell the story of when I was 5 years old and asked her why, if we were capable of cloning, we couldn't have Star Trek style replicators. I think that was probably a pretty solid early sign of the nerdiness to come. Earlier than that maybe it was my obsession with bridges -- when I was two I was practically inseparable from a little black and white book of bridge blueprints I'd gotten for my second birthday. I am still quite fond of them, especially suspension bridges:
Ain't she just beautiful?!