There's an article briefly highlighting age-related hearing loss. In short, as people get older, they start to lose the upper range of frequencies that they can hear, and this starts as young as in your twenties. If you want to test your hearing, there's a high frequency sound here.
I wonder though, whether all computer speakers are capable of producing that sound in the first place. I'm sure there's a way to look up the high-frequency cutoff for any given speaker, but I'm too lazy to check on mine. :-P As for why older people can't hear it, there's a little info here and and here, but it appears that the cause isn't all that clear, or else isn't clearly known.
Thanks to the_xtina for the head's-up.