Why do old guys keep asking me out?

For some reason, I seem to attract men in their late sixties and it kinda creeps me out. At first, I thought it was because I look old for my age but these guys always ask me if I'm over 18. I didn't used to mind it that much but the men are getting a lot more aggressive, following me around (sometimes to my car) and trying to get my number even after I've said no a million times. It makes me almost afraid to go to school alone. Guys my age (21) aren't as creepy with me and tend to be complete gentlemen (albeit shy) around me. What can I do to stop the oldies?