There are many factors that determine when and if your eyes deteriorate, starting with whether bad vision is in your genes.
I started wearing glasses while I was still at school, shortly after South Africa first got a television service (much later than most of the world). I've always blamed television for the deterioration in my eyesight.
In my case my eyes have actually improved with age and the glasses I wear now have a much weaker prescription than the ones I wore when I was eighteen years old.
Age does play a factor though, especially when it comes to reading. Several people I know have suddenly started wearing glasses in their forties. But contact lenses are so common that I believe the number of young people with poor vision is much higher than it appears.