Why can we still hear the sound at 20 kHz and 0 dB HL when I’ve heard that hearing deteriorates in the high frequencies even at a young age? And why is the dynamic range so low?

The reason we can hear 0 dB HL is that, if you (whoever is listening) have normal hearing then they should be able to hear down to the average level for otologically normal young adults. If the listener is an older adult then indeed it is true that they should not be able to hear the sound due to presbyacusis. However, a younger adult should be able to (assuming their hearing is at or close to the average) as by definition the sound is calibrated according to the average hearing sensitivity of normal hearing young adults. The concept is the same as asking why can't an older adult hear an 8 kHz pure tone at 0 dB HL.....it is just that the age at which presybyacusis is apparent is much lower for high frequencies like 16 and 20 kHz.

The dynamic range for the transducers (e.g. headphones and inserts) is less for the high frequencies. It is variable for different transducers. The effect might be slightly different according to the specific make and model in question but this is the reason why the extended range falls off quite quickly. We would be happy to look into the specifications for your model on request.