DJ, you may have not been fully matured by 18.
This is quite possible.
For instance, my real starting weight for serious weight/strength training is suspect because I was a starved tennis player. I was 6'1" and 156-159 pounds at approximately 12%. I got up to 193 at 12% with constant eating, no cardio and a perfect environment for training: college. I ate, slept, ate some more, hooked up with my girfriend, drank Myoplex like it was going out of style, slept some more, etc.
However, I didn't party with my friends, didn't stay out too late, didn't miss a meal, etc.