ashley,
Sorry to hear about the accident and the aftermath. Most of the professionals here would have to agree that this is not just a teen thing as well. People in general do not wear their seatbelts. There have not been too many times that I have had to unbuckle a dead person.
I speak at the local schools about drunk driving, drugs and seat belt usage. I had one male student tell me he would never wear his seatbelt because he had a friend who didn't wear his seatbelt and was ejected from a crash. Some moron told the kid that it was a good thing he wasn't wearing his seatbelt because if he was, he would have died. This is where sometimes not saying anything would be more appropriate.