And N is far too older compared to the rest of BW quartet.
That isn't an excuse. First we don't exactly know his age. Second, the BW group is the group to have gotten their dexes at the oldest age yet. And Red and his group are probly 19 by now. Age really isn't a factor. The real factor is how this role would affect the portrayal of his character. As I've said, I'm certain Kusaka can handle it in a believeable manner if he feels it's the right way to go with him.