Considering that the reduced hearing you are referring to, is due to aging (aka presbyacusis), a loss of 35 dBA and 50 dBA is considered acceptable for lower (500, 1000 and 2000 Hz) and higher (3000Hz) respectively.
In case of loss higher than the figures stated above, an experienced pilot may be considered fit in case he has 'normal' hearing against a background noise akin to cockpit noise. This can be tested by speech discrimination lab or may even be conducted in flight.
hope it helps!