Teenagers...When is "Old Enough"?

Well, my daughter will be 15 in March and the question around our house these days is when is she old enough? When do you let her ride in a car with a driver under the age of 20? When do you let her go over to a boy's house to watch a movie or hang out? When do you let her go on her actual first date? When, When, When, When???? Well everyone has their own opinions about this and me and Bradley's are not the same. I was thinking about letting her go to this boy's house Friday night for a few hours. She has gone to school with this kid for several years now and I somewhat know is mom. Wow, didnt know Bradley was going to be so upset about it. Its just really frustrating when I am not totally clear on what I should do. Should I let her go only if there are other kids there or should I just let her go (after a long talk with her) or should I wait till she is "old enough", which all goes back to when is "old enough." I am so frustrated with this and am not real sure what I should do. I always think, as do the rest of my family, will it happened to her like it happened to me so therefore I should keep her sheltered till she is older? But it could happen then too so do I ever let her go out of the house? I really dont want to have to make the decision now.... Why cant she just be 9 again?