Mod parent up. I started to find my monitor blurry despite wearing strong corrective lenses when I was about 40 or 45. It's hard to say exactly when because I didn't really notice for a while. Reading glasses don't really help for this if you are nearsighted, because the monitor is too far away--you need nearsightedness correction. But the correction of your regular distance vision lenses stops working for near-distance vision when you get old enough to develop presbyopia.
There are a number of ways to address this. You can get a pair of work glasses (I have a pair) that are a weak version of your regular distance vision prescription. This will allow you to clearly focus on a screen that's 24" (or whatever) away. Measure how far your eyes are from your screen with a tape measure and bring that measurement with you when you go to the optometrist.
Bifocals are a poor solution for this problem, because you really want your whole range of vision to work, not just the bottom half. However, you can get multifocus contact lenses if you wear contacts. These are a bit different than bifocals, and take some getting used to, but apparently work very well (I haven't tried them--I'm reporting what a friend who swears by them has told me).
Another thing you can do, as others have suggested, is get a really big monitor. This doesn't work as well as you might like with every operating system. I'm having great luck with a 40" samsung 4k display and a Chromebook, because ChromeOS does a stupid but effective hack to make it work: they tell the browser the resolution is half what it is, and use bigger fonts. My experience doing the same thing with Ubuntu has been less positive.