Average blood pressure for an 18year old male?!

One would have to know your general physical condition. (athlete, runner....etc.)
Teenagers typically have lower blood pressure given their youth and vigorous physical activity.
Athletes generally have very low blood pressure.
The norm for average people is 120/80, although it is often said that these parameters should be lowered.

That's pretty low considering the average blood pressure is 120/80. You may want go get examined by a doctor. Low blood pressure may lead to some serious heart conditions or even heart attacks if your heart beats are to weak. Your extremities are also may swell up or become numb due a poor circulation of blood back to the heart. You may also get light headed a lot or even black out. Of course you may also be healthy because not everyone is the same.

Its bad from the textbook point of view.