“I think the fact that he talked to the police for hours without representation is an indicator of his cockiness, not of his youth or inexperience.”
Essentially all young defendants do this. Lots of older ones do too. This is something that drives defense attorneys bananas – everybody thinks that something positive can happen from talking to the police. The only people who reliably exercise their Miranda rights and don’t talk to the police at all when in custody are lawyers and people who are well trained by lawyers (like the Mob).
In law school, I volunteered with an organization that provided suspects with free pre-arraignment lawyers (the right to a public defender only kicks in at the arraignment). I was taught that “SHUT UP SHUT UP SHUT UP” is the most important thing to say to 100% of clients in custody. We knew nothing about any client or any case, but “SHUT UP SHUT UP SHUT UP” was always the right advice. And all the clients needed to hear it, including lots of people who’d been around the criminal block before and ought to have known better. There seems to be something in human nature that makes us want to talk when we’re in trouble.