The "Posterior Subcapsular" type of cataract is apparently apt to form relatively quickly and be difficult to see; my wife went through 2 years of fruitless exams and glasses before figuring out the problem. Now she has her eyes back; thankfully.
(Exception: If someone's vision is 20/20 uncorrected and they don't have any other eye concerns, this probably isn't needed… until they hit the all-but-inevitable presbyopia, likely around age 40-45.)
But I have a question. Is it just the monitor? Or do you notice the same thing with daylight? Room lights?
Older LCD screens primarily used CCFLs for backlights, and these were notorious for going dim after only a couple years.
All modern screens use LEDs for the backlight which are much more robust but still get dimmer over time. I know that my screens certainly have dimmed significantly since they were new, but not so bad as to be hard to read.
Environment plays a huge factor as well. All screens appear dimmer in the brightly-lit room as opposed to a dark or dimly-lit one.