At 15 years old, I dont' think you could, maybe at 16, but not at 15
and just to let you know, 15 and 19 is not illegal, yes its not the ideal age gap for someone that young, but it's not illegal
i would maybe suggest talking to your parents and talking to a lawyer to see what must be done if you really wanted to move out. You would have to through courts anyway, and there would have to be some reason why you would need to be emancipated, not just because you wanted to move into your boyfriends house. It would have to be something like abuse and things along those lines.