I do light exercises like running 4.0 mph on the treadmill and my heart rate is always in 190-200, why? ... i am 19 years old

Many possible causes: Most common reason is being out of shape. When body isn't accustomed to activity, it struggles to keep up. Persistent &gradual increase in activity results in better conditioning &slower heart rate. Other causes are caffeine &other stimulants, dehydration, hyperactive thyroid, abnormal heart rhythms, &dysautonomia. These usually cause higher than normal heart rate at rest too. Talk to your doctor.