My resting heart rate is 120 bpm. Is this a normal thing for a 18 year old?

High. A resting heart rate of 120 BPM is abnormal for an 18 year old. The most common cause for a high resting heart rate in an 18 year old is anxiety, but there are several other important causes. A few of these include cardiomyopathy/myocarditis, hyperthyroidism, infection, severe anemia, or cardiac arrhythmia. If your true resting heart rate is 120 bpm, you should see your doctor.