I was like 9 or 10 I think when I first took it all the way back. Except that I always remember that as a kid in like 2nd, 3rd, 4th, and 5th grade I'd pull it back and walk around school because it felt nice to have my raw head bounce around my boxers. I remember though in 5th grade (9 or 10) that I pulled it all the way back and it felt really nice. But like I always remember having to pull it back and wash out smegma. So for me, its kind of hard to pin down.
EDIT: I remember now that I pulled it totally off the corona in 5th grade.