Most noticeable hiss is well within the audible range of those in their 50s or even older.
Like I said before, the correct answer to your question is the same as for "How long is a piece of string?"
That is, you have to measure the specific noise / piece of string that you have.
If you're interested in experimenting with the perceptual effects of noise, the Audacity program is a powerful (and free) toolkit.
You can easily generate noise, then experiment with filtering out various parts of it to see which parts are perceptually most noticeable.
You can use white noise to generate equal levels at all frequencies, or pink noise to approximate the spectrum of most music.
For example, generate a few seconds of white noise. Now apply a 10 KHz low-pass filter to some of it. While the displayed waveform will look very different, you'll notice little difference when you listen to it.
