I actually can't remember how old I was when it started. As a kid my mom mentioned periods in passing and since then I never really thought about it until that fateful day.

It actually took me awhile to figure out what was going on and since then it was just part of the routine. I guess I don't remember exactly when it happened because it never really meant anything to me, sure I knew what getting a period meant, but it was mostly just an inconvenience.
But like Gnome (I like the name btw

) said as time went by the cramps got worse. It reached the point where it was debilitating. But as I found ways of coping with them it became more bearable. So I'd only be knocked out for a few hours instead of a whole day, and now it's only bad the first hour or so, and I can work through it.