Keep in mind that most military applicants are older anyhow (many are 30+), so I think you're overstating the importance of your age. Of course, if you were a 30+ IT engineer, that would be different story. In other words, age matters more for those who are bankers, consultants, engineers and other "traditional" MBA applicants.
Also keep in mind that b-schools love military applicants - probably more so than any other applicant. They believe that you have extensive leadership experience, which is something no one can really teach (but only can learn through experience). It's easier to teach someone accounting, finance, marketing, etc. than it is to teach them how to develop strong interpersonal and leadership skills.
So what you have to really demonstrate in your application is this:
(1) That you have the analytical abilities to handle the academic rigors of b-school. They know you are probably good with people - the question is, are you good with numbers and analysis? Since your GPA is spotty (but the flipside is that it was ages ago), you really have to ace the GMAT. If you're in the high 600s, you should be competitive for a top 20. However, to be competitive for a top 10, you should really get above 700 given your GPA.
(2) That you know what it will take to transition from a military to a civilian career. In plain English, they want to know that you know what aspects of your leadership experience are transferrable to the b-school environment and to the civilian sector (and what aspects aren't). They also want to know that you have a pretty good idea what it is you want to do post-MBA -- that you've really thought through what it is you want to do as a civilian, and what it will take to make that transition as successful as possible. This will all have to come through in your essays as well as your interviews.