As a freshman in NROTC you would NOT apply for a sideload scholarship. You would re-apply for a 4 year high school scholarship which would, in effect, be a 3 year scholarship if you were awarded one. This is something fairly new which started last year. While AROTC awards walk-on scholarships, NROTC does not. All NROTC scholarships, including sideload if/when you get to that stage, are national competitions decided by national boards. If you're having age issues when you start school I'm not certain you'll be able to do NROTC as a college programmer that year. I think there is still paperwork that parents need to sign. Double check with the unit at the college you plan to attend regarding this as my memory is a bit hazy on this point.
EDIT: You can certainly take the academic classes that a NROTC freshman would tak e without being enrolled in the program. This would help bypass any signature issues and at least get you started on the path, and avoiding doubling up in a later semester.