Well, I've had glasses/contacts since I was in first grade due to double vision. I don't know the scientific term for it, but basically one of my eye muscles doesn't function the same as the other, so it focuses on things wrong. I never really understood why glasses help but they do - force the eye to point straight instead of crooked. I even had an eye operation to fix the muscle but it didn't help. I'm not really sure why the problem would develop later in life.
