It is normal for vision to get worse as you age. For myopic or near-sighted people like me, we see closeup naturally, so the N900 is easy and it's not getting much worse as time passes. Most people, however, get more farsighted and need glasses to read closeup over time. Of course, eyestrain can accelerate the process but I don't think it's a major factor in most cases.
In the browsing and its where I'm having problem, stting the text size to large is lot helpful so others can set it to large or extra large to suit ones need..