Just came across the fact that people over 25 usually cannot hear above 15 khz.

So if you are meeting an 18 year old subject her to the 15 khz sound and ask her if she hears anything?  If she does then she is 18-24.  Most undercover agents are 25+ so if she can't tell you when you are making the sound maybe set up a hidden speaker then she is lying and must be a sting!

Also shooting ranges would impact high frequency hearing and also a good way to test for high frequency hearing loss likely to be LE.