I think it's fine to write about it if that's where your desire to enter medicine came from. Be sure to include anecdotes from college that illustrate why you're still interested in medicine or how you came to further realize this was what you wanted to do, etc.
i had an important experience for myself from the summer before college - and i think it works ok. but yeah - i would write about recent stuff as well. i'm not sure how far back i would dig though - 9th grade might seem a little dated.
If it's the reason, then it's no problem at all to put it in. They're looking for the motivation, whether that came recently or a few years back. No big deal.
I know one of the things that influenced my desire to become a doctor is my father's battle with various health problems. He's had both cancer and a serious head injury prior to me entering college. Did you also have some experiences in college where you furthered your interest in medicine?
