Question about ADD and ADHD?

I am a 19 year old college student and all my life I have been having a hard time concentrating in school. I dropped out of high school because I could never pay attention in class. Now in my second semester in college i feel as if it is all going to be a repeat soon. When I read i start dozing off and day dreaming about something or when taking finals I stop and think about other things. When professors are talking I pay not attention because it is so hard. I failed my last final because I could not concentrate throughout the whole thing. It takes me an hour just to start my homework and when actually doing it, it take me forever just to complete it because I am constant stopping. Now one doctor told me I am to old to be medicated on aderall, is this true? I have always felt like I have had add or adhd either or all my life but I was just a quiet kid that never spoke up.