If you can't identify hospitals in Miami, and call or look at the web sites of each to find out their age requirements, then you are too young to volunteer.
Community Service and Volunteering by Teens: How to Find Opportunities
Very detailed information that discusses your many, many options - the usual and many activities you probably never thought of. Includes instructions on how to find specific volunteering opportunities (with animals, with children, just for one day, volunteer vacations, etc.).
You are NOT too young to volunteer!
Ways you can help, no matter how young you are
Includes ideas for volunteering to help animals.