Hi... I think the question of whether or not your hoped-for score (B-, really a B on your transcript) will be enough can only be answered by the program(s) you're interested in entering. From what I've seen, program entrance requirements vary widely.
About being 50: I am 44, myself, so I've spent a lot of time researching people like us who go to nursing school in mid-life. The good news is, there are many of us!
I know that I will enter the field with so much more prudence and wisdom than I would have, at a younger age. (Not to say that plenty of younger, new nurses wouldn't be just as prudent, etc.)
Study hard and go for it!
You can have many years ahead of you in which your work will make a real, tangible, meaningful difference in the world.