You'll usually stop hearing the CRT whine in your 20s, 30s if you're lucky. It's at nearly 16KHz and the highest frequency you can hear always drops as you age.
Of course, these days people also regularly don't hear it since CRTs are nearly all gone.
Even back in my youth and early adulthood, there was a wide variation on how loud CRTs were between models and individual units. There were some that were close to silent for me, while others were very noticeable.
The signal is at a frequency of one cycle per line-second, but there's an idle period while the beam goes back up to the top where it still cycles so the actual frequency will be a bit higher than a simple lines/frame * frames/sec calculation would indicate. Even for broadcast TV that was beyond what a lot of adults can hear (but a lot of kids can, as plenty of posters in this thread have said.) I want to say 15.7khz but I certainly won't swear to that. As your resolution goes up the frequency will go up so some people would be able to hear what resolution a monitor is set to.
