Found this whilst browsing a forum, basically play back each sample below until you can’t hear the sample when played. When I do it I can hear everything, as you get older you will probably start to lose the higher frequencies, I had to turn the volume up a little to hear the last few, not sure if your supposed to do this or not, but I could hear them quite well and I’m 29. The higher frequencies have been used as a ‘Mosquito’ noise outside shops in the UK to stop teenagers hanging around.
Here are the samples:
10,000 Hz
11,000 Hz
12,000 Hz
13,000 Hz
14,000 Hz
15,000 Hz
16,000 Hz
17,000 Hz
18,000 Hz
19,000 Hz
20,000 Hz
21,000 Hz
22,000 Hz
23,000 Hz
24,000 Hz
25,000 Hz
(originally taken from http://www.ochenk.com/entry.php?id=63)