The STA-21 program is only for enlisted personell. You would have to enlist in the Navy, which I now think is too risky because you really want to become an officer.
I think NROTC is the best choice for Navy, or you could do the Platoon Leaders Class for USMC (with the 28 year limit).
If NROTC is not willing to take you at your current age, what you might consider is going to Auburn, not enrolling in NROTC until after the end of your sophomore year. But become a sophomore quickly by attending summer sessions. Theoretically, you could become a junior by the fall of 2016, if you start this summer and also attend summer of 2016. Then join NROTC for your junior and senior year, and meet the 27 year old limit. Graduate in Spring of 2018.
This could be tricky. The summer of 2016 would need to be “mini-semester” so that you could attend a 6.5 week training course in Rhode Island afterward. For “mini-semesters” see Auburn’s summer schedule for this year.
http://www.auburn.edu/main/auweb_calendar.html
Not sure that would work, but it might.
Actually, if you are not on scholarship, they don’t need to be all that high. I think 2.5 GPA. Physical fitness is also very important, especially for the Marines. I hope that you like running.
If all else fails, you can apply directly to OCS about the time you graduate.
http://www.ocs.navy.mil/ocs_program_requirements.asp
The 6.5 week course mentioned above is an abbreviated OCS.