Generally, I feel like childhood is what shapes you as a person the most and usually that's where you solidify where you're "from." I know if I ever moved anywhere else, I'd always be a Bostonian in my heart.

But my uncle fits that criteria, so I'd say they're also a Bostonian as long as that's how they want to define themselves.

He moved for college after my mom / his sister did the same, and has stayed for 50+ years after that. He's one of the most passionate Boston sports fans I've met, and has gained our accent over the years. Not a heavy Boston accent by any means, but there's no way anyone would be able to tell he grew up in South Carolina. He sounds like he's from here and would call himself a Bostonian.