I didn't choose a major until winter quarter of my Junior year, when the registrar threatened to put a hold on my class registration unless I chose.
I didn't start doing medically-related ECs until Spring quarter of my Junior year, because I didn't know what the heck I was even interested in until then.
I didn't decide I wanted to go to medical school until eight months after I graduated.
I'm currently in my second year at one of my top choices for med school.
Basically, so long as you focus on your goal once you've figured out what that goal is, it's never too late. You might consider taking time off, not so much to pad that application, but so that you gain the opportunity to learn and experience some stuff before you pack off for med school. Really, a year off goes by faster than you could ever imagine, and I cannot fathom how people manage to interview while they're still in class and still doing their ECs. It's gotta be rough.