When I was looking to get a car back when I was 16 or so my dad said this to me:
"Sure son you can get any car you want, I'll even help you pick it out and check it over. Just tell me when you have the money and well go look."
When of the best things he could have told me. I worked hard for my first $4k Toyota and didn't beat it because I made that $4k sweating cutting lawns and at an $8 an hour job. Then I got tired of it and started buying Mercedes.
I feel that every kid should be forced to buy there own car, it teaches them real world things. Like money doesn't grow on trees, and not to beat there cars because if they break it, they will be fixing it. I will say the exact same thing to my kids when the time comes. I don't care if I could buy them a new BMW, they didn't earn it so they don't get one.
But on the other hand I know a few people who were just given cars when they turned 16 or 17. Lots of 3 series and A4's were in the highschool parking lot. Said cars seemed to get trashed and in accidents rather quickly...