I agree with sarah, definitely address both topics, why medicine and especially why did you decide now. I dont necessarily think you need to focus on it being so "late in the game". Late in the game is deciding you want medicine after having a different career for 15+ years (and you're in your 40s) If you're deciding in your senior year of college, you're still right out of the gates. Just think of it has having taken your own path. They would much rather have someone who came to the decision naturally, than someone who started pursuing it years before and can't even remember WHY they want it anymore.
Since it is something you decided in your senior year, I'm guessing there was an experience you had that led you to it? If so, definitely right about that. It's important to be specific, just like telling a good story, lots of details etc. Maybe it was a volunteer thing, shadowing, etc. remember, try to avoid phrases that state the obvious... you want to say what attracted you to the field of medicine without stating "I'm really attracted to medicine because...." Convey your enthusiasm and passion. Write about it from the heart and you'll do great!
Good luck!