I'm very young but I have this horrible fear of death/dying, what do I do?

I have this terrible fear of death. When I see old people like on the subway or in other public places I get so depressed because I know they will die soon and I fear getting to be like that myself someday. I fear it so much that I avoid anything that I think might lead me to having a shorter life. I'm a total health freak. I eat as healthy as knowingly possible, I work out, I take all kinds of vitamin/mineral/dietary supplements, I never drink or smoke. But I still know that no matter what I do I will most likely get old and die someday and it makes me very depressed. I don't believe in heaven or any fantastical kind of "after-life" because it just doesn't seem logical to me. I'm a college student and a science major. I honestly don't believe there's anything after death...I think its like when you sleep but you don't dream. Just nothingness. What should I do? Its starting to make me paranoid and depressed.