Milton Friedman, RIP. See also Tyler Cowen. Free to Choose was the first book I read in the human sciences, the first serious book about politics or economics I ever read, and obviously had a huge, transformative effect on me at an early (11 or 12 or so) age.

Update:

Quoth Ogged:

On the occasion of Milton Friedman's death, Jacob Levy brings out the big guns in the ever-ongoing precocious nerdosity contest. [...] I am slain. As a big Doors fan, I wanted to find out about this Nietzsche character who Jimbo loved so much, so I read Thus Spake Zarathustra in 8th grade, during which I was what, 13? I think we should set aside questions of whether we understood what we read, counting books just as long as we believed that we understood something of them. Can anyone beat Levy?

Aw, heck, that's not even the nerdiest thing I did when I was 11 or 12-- ask me sometime what I wrote in sixth grade. I'm not going to blog it, though. It turned up in an old box when I was packing to leave college; my housemates got at it and read it and I've never lived it down.