[blincoln]

They weren't that high frequency. I could hear computer monitors into my twenties at least. I'd guess somewhere around 20 - 22 kHz. CRTs were largely replaced by LCDs by my late 20s/early 30s, so I don't have a good sense of when I stopped being able to hear frequencies that high.

[pezezin]

VGA monitors had a minimum horizontal frequency of 31 kHz (480p at 60Hz), way outside the human hearing range.