Blunt force trauma is pretty hard on the body. I can definitely see how getting run into by a car could cause someone's heart to stop, even someone young.
My mother-in-law died of lung cancer, but not really. Really, she died when her heart stopped beating, due to the sheer exhaustion of trying to keep her lungs working as they filled with fluid. So, what really killed her? I still say it was the cancer, even though she was overweight, diabetic, a former smoker, and at high risk for a heart attack.