Mosquito tones are high-frequency tones (17kHz and higher) that are inaudible by adults but can be heard by teenagers. It is very normal for people to lose their hearing as they age. The loss of hearing is gradual in most people, while other may experience more severe hearing loss.
Let's see how young your ears are. Here's the list of tones that Russ was playing over the air. How many can you here?
From NoiseAddicts.com - Hearing Test – Can You Hear This?