I just read that if your blood pressure is 120/80 is considered prehypertension. I thought 120/80 was optimal. Should I be concerned?

120/80 and up. An optimal blood pressure is less than 120/80. Anything from 120/80 to 139/89 is prehypertension. 140/90 and up is hypertension. Eating a healthy diet, maintaining a healthy weight, exercising, and cutting back on alcohol/smoking are great ways to make sure your blood pressure stays in the healthy range.
No. Listen, that blood pressure is fine. However, it won't stay fine if you smoke, drink too much, eat crap, and don't exercise. Keep it there and you should be fine.