DanDMan: I think for me it was when I was 11 or 12 when I first tried measuring my penis. I was too scared of getting caught with a ruler in the bathroom since I lived with my folks in a really small apartment. So I used my hand, and my penis was the same lenth as my hand lenthwise. This was really innacurate but the measurement came out to be about 5.5 to 6 inches. Looking back, its not bad for a 12 year old.
Now ten years later I've only grown a little over 2 more inches but much, much thicker. The girth came suddenly if I remember correctly, one day I just realized it was much thicker than previously.
From then on being big was a pretty neat thing. :-)