Not to make your speech harder but I don't think your 2 points really work in this situation. You're saying you want the Marines to be able to recruit people under the age of 17/18? What about their ability to finish high school? Marine recruiters don't seem to be having any trouble meeting their quotas right now so the age change wouldn't really do much. As for PTSD, how would having even younger kids go help with that? They'd be even less able to deal with the trauma because they'd still be going through puberty and other such emotional issues. I can't honestly think of any reason to lower the age more than it is.
ETA: Okay but your points still don't make a lot of sense if the age was raised above 18. For one, how would that help recruiters reach their quota because raising it above 18 would cause them to lose all those candidates who they're getting between 18-21 (I guess I'm assuming you mean raise it to 21 because otherwise it's not a significant enough of a raise to mean anything). As for mental issues, it really doesn't have a lot to do with age at 18-21. Sure younger people may be less emotionally equipped to handle some of the stressors the military can give you. However, Marines/Soldiers/Sailors/Airmen of all ages can get PTSD. It depends entirely on their experiences and their own ability to deal with those experiences and I know of men/women in their 30s and 40s who have just as hard a time dealing with death and trauma that war will bring as those in their teens and 20s. Honestly I think the age is fine as is. Sorry since that doesn't do anything for your speech but there are a lot of 18 year olds who have really benefited from military service and I feel they are old enough to choose to join.