I've worn glasses since I was 8 (and at the time my vision was 20/200 (equiv to -2) or so, so I probably could have used them by kindergarten or 1st grade). My prescription usually got worse by .5 to 1.5 D per year, depending on the year, until I was about 22 and my vision stabilized. My current vision is 20/850 (-8.5) with an astigmatism of about 1.5D on top of the myopia; I have only met a couple of people with worse vision than mine. That said, my vision has never been a major issue in my life.
As a potential alternative to glasses, I got my first contacts when I was 11; with disposable contacts as an option I would be open to letting a kid as young as 8 or 9 try them with the approval of the optometrist.
(Yeah, this is a month after the original post, but I figured you might still be interested.)