Being 17 at the academy is not uncommon. Many cadets don't turn 18 until BCT is over; or even going into Sept-Nov. The youngest age to enter military service; either enlisted, academy, or JrROTC is 17 years old. They WILL NOT send you to the prep school because you are only 17 years old on entrance day.
If you get an offer to the prep-school, instead of a direct appointment, it simply means that overall your application was great; but there was one or more areas that probably needed some improvement. Usually academics. E.g. 3.7 gpa, but every class was a 4.0, except for math, which every year you got a "C". Or maybe you had a 3.9-4.0 gpa, but every ACT test you took, could only get to around 24. Or maybe you were one of these brainiacs that had all the academics, but because of family or community limitations, you never had a chance to get ANY leadership experience. There's a lot of reasons to get a prep-school slot instead of a direct appointment. And if any of the above; and plenty of other reasons exist; plus you happen to be a recruited athlete; then that's a "BONUS". It's like having a "Red-Shirt" year. Not that that is an objective, simply a bonus. I.e. An applicant with an exceptional application, with 3.9+ gpa; 29+ ACT, numerous sports, leadership, clubs, community, bla bla bla..... is not going to get a prep-school slot because they are being recruited for athletics. They'll get a direct appointment. MOST Recruited athletes get direct appointment.
The point is, there are many reasons a person would get a prep-school offer instead of a direct appointment. Being 17 years old is NOT one of them.