I think that positions Luffy in a really interesting spot because he has been shedding his life span throughout the manga, like in Impel Down.

That was really only a thing in Enies Lobby and then Impel Down. I don't think that will be a factor for Luffy. There's just too much child of prophesy stuff riding on him finding the One Piece and being the guy that Joyboy and Roger were waiting for.

As for why Roger laughed, I don't think it was because he was too early and wouldn't be able to fulfil Joyboy's wish. I subscribe to the theory that Luffy's dream is to throw a party for the whole world. The One Piece probably plays into that somehow. Like maybe its some instrument that allows everyone in the world to hear it, or maybe the original Binks Sake or something like that. Something that is goofy and not the traditional treasure that pirates would lay their life down to find. Hence, its funny and that's why Roger wishes he lived in the same time as Joyboy.