What would it possibly mean when an 18 year old's pulse rate is at around 120?

Abnormal: A resting heart rate of 120 is not normal. The person is either extremely unfit or suffering from cardiac rhythm disturbances or hormonal imbalances such as thyroid dysfunction. They could also have anaemia which causes the heart rate to elevate in comparison. You need to see a doctor to undertake blood tests and Electrocardiograms to rule out any worrying pathology

Heart rate: A healthy adolescent has a heart rate that responds to many things. If an 18 year old is asleep and his/her heart rate is 120 that could be a problem.

It is high: For an 18 year old to have a pulse rate of 120 at rest is too high. Possible reasons include anemia, hyperthyroid state, poor fitness , respiratory problems, infection, fever, dysautonomia, conduction abnormality, and many others. I recommend you have a thorough medical evaluation to determine the reason for the high rate.

Depends: Tachycardia is usually a result of exercise or increased metabolic needs, lack of oxygen in the blood, high blood pressure, stress, hyperthyroidism, dehydration, acidosis with compensation, and many others. You need a history taken, a thorough physical exam, and some blood work among other diagnostic tests to begin to find an answer to such an open-ended question