A couple of years for me.
I first started playing D&D in 6th grade, with the red box rules. It wasn't my agenda or anything, and it's not like I wasn't willing to try other games...it's just that most eleven-year-olds don't have the resources or opportunity to do so. Especially those of us who lived in small rural towns in the American South.
Once I was in high school, someone invited me to play a game of GURPS (it was the height of the Satanic Panic, so non-D&D games were the only ones that a lot of us could get away with playing). GURPS was popular at my school because while it wasn't technically D&D, it was generic enough to be played like D&D and didn't use "funny-looking" dice that would get us ratted out.