Depends what you count I suppose. I was creating bat files and generally messing with my PC pretty soon after I got it, I think I was about 15. I had an Amiga 600 before that but at the time was mostly focused on writing and gaming.
The first code I tend to count was learning HTML when I was 17. I was probably using Windows 3.11 or 95 on a 486. I learned CSS and PHP a couple of years later; noting CSS was very much the big new thing at the time :) I didn't really embrace Javascript until years later.