How can I convince my parents to let me buy a dirt bike?

[Evan]

I'm a 13, 3 months away from being 14, and my parents won't let me buy a dirt bike, even though I have enough money for a nice use 125cc, that I have saved up for with my own money (not coming out of savings account-christmas money-allowance-babysitting). I'm coordinated enough to ride one easily, and I live in the city BUT we have a cabin and there are lots of people that have them. They let me go to a camp that had dirtbiking as one of the daily activities (which i did), and I got moved up to the intermediate group by the second day, but now they won't let me buy one even though I'm not going to go super fast or hit any jumps with it. I just found out about the MSF-dirt biking course, and I told my mom I'd take the beginner one, even though I know it all, and the intermediate one, just to show her I'm responsible enough, and all she said was I'll think about it. The real problem is, that they're both doctors, my dad a pediatrician, and my mom a family doctor, so all they hear about is injuries. But, they've never had a child get injured on a dirt bike, so they use my mom's example of a man who had his ATV fall on him, and break his neck, even though it weighs like 4x more than a dirt bike. Do you have any ideas about how to convince them-stats, reasons, etc...?