I think its a great goal for a kid to set even in 8th grade to want to attend one of the service academies. That said I can only think of 2 reasons parents would allow their 12-13 year old kid to commit 9 years of their life to the military.
1) They are insecure in their kids academic/athletic performance and feel she has peaked already and the more the coaches see of both the less likely they are to want her.( I get why some kids commit early if they get a great offer and feel that money may not be around in the future, obviously not the case here as the offer is always the same)
2)The parents are egocentric to the point of mental illness. Why not wait until your child has a little more life experience under their belt before making that decision. I know I will get crushed for this but add in the fact about some of the recent reports on how females are being treated and there are some real concerns.