"Starting around the age of forty or so, the ability to focus on closer objects decreases - books and newspapers have to be held farther away to bring them into clear focus. This is probably the first sign of the condition called presbyopia (from Greek words meaning "old man's eyes"). Another sign of presbyopia is that people's ability to refocus quickly between near and far objects decreases."
This becomes painfully apparent when Bon brings something to me and says "Here, look at this!" and puts it six inches from my nose. I used to be able to focus that close, but now find myself taking whatever it is and moving it out to 16 inches or so. Sigh. I'm getting old man's eyes.
quote from http://www.ccohs.ca/oshanswers/ergonomics/office/computer_glasses.html