Good question! 4000Hz is where we typically look for and see noise related damage. This is because the ear canal naturally amplifies this frequency. Although we evolved to amplify natural sound, this has backfired on us with mechanical/music sound etc and we end up amplifying it to the degree where it causes damage. A person can have normal hearing before and after the 4000Hz region, with a dip that starts at 3000, troughs at 4000 and recovers at 6000/8000.
Age related damage starts high and works its way down. Kids start out being able to hear above 10,000. This is lost first, then the hearing test will start to show loss at 8000, 6000 and so on. This is because the high frequency end of the cochlea is damaged first through wear and tear.
Two different mechanisms, both lead to a high frequency loss.