My parents refuse to believe Im an Atheist. Help?

My mother (a devout roman catholic) and my father (a christian) have just refused to believe that I am atheist. I'm 16 and my mother says I'm too young to be making these sorts of decisions in my life, while my da just thinks I say I am to be "cool". My family lives in the bible belt (both of my parents were born here too) so its hard to find other atheists, but I'm getting fed up with my parents acting like these. Do you guys have any advice on how I can show them or tell them that I'm old enough to choose my own religion and that I'm being sincere about it?