Answer: Between 20 Hz and 20,000 Hz.
Sound waves are formed by compression of air. The air waves enter into the ear, vibrate the tympanic membrane which causes the three ossicles to move in response. That physical response causes the oval window to vibrate, which transmits the signal to the fluid inside the cochlea.
The cochlea itself has several hair cells along the Organ of Corti. These hair cells are responsible for converting the physical stimulation into an electrical signal that is then passed into the brain via the auditory nerve, a branch of cranial nerve VIII.
The hair cells are tuned to respond to different frequencies. Higher frequency tuned hair cells are at the base of the cochlea (closest to the oval window), while lower frequency tuned hair cells are at the apex.
Loud noise can physically destroy the high frequency-responding hair cells. By old age, many people lose their ability to hear high pitch tones. 16,000 Hz is often the highest pitch that older adults may be able to hear.
Try bone conduction headphones. They bypass the activation of the eardrum and transmit vibrations directly to the ossicles.