Who is at fault if an underage driver with a permit and no adult present aggressively braked and the driver behind hit the bumper of the car?

If it went down just as stated, both of you are At Fault. You know you shouldn't be driving without an adult present. That's a violation of the law. The other person rear-ended you and could be charged with "undue care and attention" or "following too closely" or the like. If the damage was minimal and no insurance info was exchanged by the other person then bye-bye and you probably won't hear much about it. However, if the police were brought into it or the other person reported this to their insurance, then you may be cited and fined (and a "mark" will be made on your driving record). The other person may also be cited and fined as well, and any damages to the vehicle you were driving will be paid by the other person's insurance because they were at fault for the accident.