My step-son attempted to enter the U.S. Army three years ago and we were told that some study had linked ADHD in adolescence to suicide while on deployment or some such thing. He was not allowed to join. However, they called him back a year later wanting to try again. What I will say is that the standards seem to change a bit over time. What one time they would not allow in, now they are allowing with a waver.
The simple fact of the matter is this. If you have your heart set on becoming a Marine or one of the other military branches. Found out what their standards are and do the best to meet them before you get in. Stay clean morally and physically, educate yourself and pass the ASVAB and graduate high school. If you can do all that I am certain you will have a good chance of joining the Marines. Good luck!