The middle checkpoint
Hearing age tests step through the top octave because each frequency draws a different age line: 8 kHz nearly everyone hears, 10 kHz most adults, 12 kHz is typically the boundary of the 50s, 15 kHz of the 40s, and 17-18 kHz of the mid-20s. One tone proves little - the pattern across the ladder is the measurement. This page is the 12 kHz rung; the hearing age test automates the whole climb.
In music, 12 kHz is deep in brilliance territory: cymbal decay, the finest edge of vocal sibilance, synthesizer sparkle. Its loss is subtle in daily life - noticeably less costly than losing 10 kHz, noticeably more real than losing 16 kHz.
Testing honestly at 12 kHz
Two confounds matter up here. First, hardware: most decent headphones handle 12 kHz fine, but laptop and phone speakers get unreliable, and heavily compressed Bluetooth connections can shave it. Wired headphones make the test honest. Second, level: turn the volume up gradually from silence rather than starting loud - at these frequencies you cannot judge loudness well, and blasting a tone you cannot hear is exactly how the remaining hair cells get hurt.
If 12 kHz is audible but 15 kHz is not, you are in the statistical company of most 45-55 year olds. If neither is audible on good headphones, run the full hearing test to see the shape of the curve below.