I worked in audiology for my entire 30's and tested my own hearing (biocalibration) once a week.
Never once saw a reduction in my hearing thresholds (250Hz - 8kHz)
Nor have I read any supporting documentation that agrees with a reduction of 8kHz thresholds before the age of 55.
http://www.roger-russell.com/hearing/hearing.htm
http://personal.cityu.edu.hk/~bsapplec/frequenc.htm
https://www.researchgate.net/publication/261767650_Extended_... (table 2)
I worked in audiology for my entire 30's and tested my own hearing (biocalibration) once a week.
Never once saw a reduction in my hearing thresholds (250Hz - 8kHz)
Nor have I read any supporting documentation that agrees with a reduction of 8kHz thresholds before the age of 55.