As a former Marine that enlisted at about 130 lbs, and graduated at 119 from recruit training, I strongly discourage the usage at all before enlisting.
What I see people coming to T-Nation looking to enlist is failing to realize that the military is more about leadership than it is strength. Sure, strength does have its place in the military. But, if you're a strong leader, you can get anything done. That doesn't mean having 20 inch arms, that means being able to convey what needs to be done and accomplishing the mission with what you have available to you.
I'd strongly suggest for you to put your cycle on hold until at least your 2nd enlistment as you'll see the picture of a Marine is not what you may think of them when you actually get in the fleet. Marines come in all shapes and sizes, but it's the characteristics that they bring to the table that's the most important.
Others will say, '18 this, 18 that, you ought to be 25.' I don't care. You're adult enough to decide. This is just my advice.
Develop your mind and attitude first and foremost. And, good luck.