Rob, you can exhibit ADD like symptoms secondary to a brain insult. Secondly, it is possible you have ADHD or ADD and were never diagnosed. However, if you don't have ADD or ADHD by the age of 17, than in answer to your question, you cannot develop the primary disorder later in life.
It might be possible if you have suffered some sort of head trauma..
Otherwise, you can only go misdiagnosed since childhood...
No, but it can persist into adulthood. There are tons of information about ADD. Seek and you shall find.
Other people asked questions on similar topics, check out the answers they received:
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them