I've been in front of screens a huge number of my time during most of my youth and still had 20/20 for both eyes. At 25 my sight started decreasing and now I need glasses if I want to look at something in the distance. I have two theories about this, it's either my genes, or it's the fact that I started using a macbook pro with full luminosity at all time (even during the night) when I was 25.