Its not uncommon to be diagnosed later in life.

I knew a man that was diagnosed in his 40s and had found out what his problem was all his life and went to college and succeeded in life for the first time he was ever able too.

I was diagnosed when I was five, so I was really young--but its possible I only have ADD issues and its really just my Autism, but I still have both diagnosis of Autism and ADHD (I was a hyperactive child--still am in my adult life, but I feel its just my manic episodes).

But, you are seventeen and its not uncommon for teenagers to be diagnosed with ADD/ADHD either. In fact, it's common to be either from a young child into your teenage years. It starts to be less common in adulthood, but as I mentioned before its not even uncommon then.