No, but speaking from experience, many will. I didn't decide medicine was for me until after sophomore year in college. By that time, my classmates had been building on the experiences they acquired in high school and their knowledge of the medical field far surpassed mine, which put them at a significant advantage. For instance, when I was volunteering to learn what medicine was all about, they were finishing their EMT-B certifications to find part-time employment in the field and start accruing clinical experience. You don't need to start now for the sake of your future medical school application, but the earlier you start the further ahead you will be when your first admission cycle does roll around.