It's obvious — except to a minority of the Supreme Court — that a juvenile being questioned by the police will feel less able to get up and leave than an adult in the same situation. 

The 5-4 ruling arises from the interrogation of a 13-year-old North Carolina boy suspected of committing two home break-ins. A police investigator questioned the boy in a school conference room, but he wasn't read his rights. By adult standards, the boy wasn't in custody, the trigger for a Miranda warning. He wasn't under arrest, the door was unlocked and at one point the police investigator told him he could leave. But common sense suggests that a 13-year-old taken to an office and faced with not only the police but also school officials won't feel free to leave or to refuse to answer their questions. In the end, the boy confessed to the burglaries