Since she was ten? Jesus. Okay, so when Dean was ten he was facing down a shtriga in a motel room, but that's different in a way he can't pin down with words but feels as acutely as a knife wound, clean and sharp. And even then he wasn't really a hunter – that took another six years, if he remembers right. At ten you're not meant to embrace the life, to aspire to this. If anything Sam's reaction to finding out, much as it pained him at the time, is the 'right' one.
That argument sort of falls apart, granted, when he tries to reconcile it with seeing John as a superhero, because what kid doesn't want to punch bad guys in the face with awesome powers and neat gadgets and all that? But still. Hunting's something you're born to, right? Not something you choose to be?
“I guess, yeah.” Dean replies; he can't keep from frowning slightly, but the fact that he sounds something other than sarcastic about the whole arrangement, that he manages to come off even vaguely sincere, is a personal triumph. “Sounds like you have it all figured out, anyway. So what, you're heading to California, hooking up with this aunt?”
He's got no idea what to say to this girl, this still-pretty-much-a-child who's convinced hunting is the best thing ever. 'Don't' comes to mind, as does 'You're an idiot' but neither's right or a good idea or quite what he wants to say on the subject.
The patties sit on the counter, ignored for the moment.