Lower your volume, put on headphones and press Start. For each tone press whether you can hear it; the test steps upward until you stop hearing and reports the frequency reached.
No. It is a curiosity tool, not a medical device and not a diagnostic instrument. Consumer speakers, headphones, browser volume and room noise all affect the result, and only a qualified audiologist with calibrated equipment can assess your hearing.
Young listeners often hear to around 17-20 kHz, and the ceiling usually falls with age, commonly to 14-16 kHz by middle age. Many headphones and laptop speakers also roll off before 18 kHz, so the equipment may be the limit rather than your ears.
Turn your system volume well down before starting and raise it only until the first tone is comfortably audible. Never push the volume up to chase a tone you cannot hear — that is exactly how high-frequency hearing gets damaged.