[Kuncher]

It's not necessarily genetic. Certain eye conditions and prescriptions can be, but the need for reading glasses past a certain age is pretty much universal.

I believe it is something to do with the muscles weakening in, or around, the eye which make it harder to maintain a focus at a short distance.

It's age related.

According to my Missus, if you stared constantly at a monitor for hours and hours a day it may have a limited affect on your prescription, but the fact is you would need to be staring at a fixed distance constantly. Many people using a monitor will not be staring at it incessantly.

She can probably explain it better than me lol given that she is a consultant optometrist for Laser Surgery.

I agree it's age related, but the specifics of how you age is genetically determined to a large degree.