Out of interest, what should it be, Bryn?
There's no real "should", ...tr..., the cilea which pick up the higher frequencies fail and are not replaces as the years accumulate. There are statistics relating to mean frequency range perception by age, but there are too many variables involved. However, once you past your mid-twenties, it's downhill all the way. I can just about make it to 24 seconds, which equates to a top frequency of about 12 kHz. Five years ago I could still hear a little above 15kHz.
There's a sort of rule of thumb chart of upper frequency perception limit by age,
here.