Was going to say 20, when I got my Cyrix PR166 as a senior in college.
But them some post reminded me, that in college, I was writing software on occasion because I was an EE major. Now I do software for a living.
Then I thought more. I had an Apple IIc back in the day. I started writing programs for it maybe when I was 12. At 13, I used my collective knowledge to write some programs. One was a cheesy drawing program for a golf game I wrote earlier. The golf game was SWEET! had sand traps and everything. In the end, I hadd 18 images for the course and 18 images for the greens. The greens had hills too!!! Used different colors to represent different breaks in hte green. It worked AWESOME! Till this day, that program is my crowning acheivment. I had to read joystick information and monitor keyboard strokes simultaneously along with having to learn how to read from the graphics buffer to get pixel color info.
I miss those days.
Now I think back prior to the apple IIc and my atari 5200. i could whip peoples buts in those games, not from practice, but because I could "see" how the programs were written. not much more than position, velocity and acceleration. I recognized those things back in the day at probably 8 years old. Even then I realized how games worked, even though i didn't know it at the time.
I don't go by the term geek though,. I'm more of a dork
So to answer your question, I might have to go way back to the age of 8. Matters how you look at it though. I was deifnietely a dork at 13 though. THat's gaurenteed!