Thank you for the opportunity to help you with your question!
I am assuming the question is about mortality meaning death.
So the major cause of mortality among all Americans is cancer which is health related. The major cause of death among young adults is traffic accidents. The reason for this is that young adults try to drive fast and are not cautious whereas all Americans generally are defensive drivers.
Please let me know if you need any clarification. I'm always happy to answer your questions.