The best thing for my ADHD has been finding the right professionals to help me diagnose and manage it. Perhaps I'm reading too much into this, but I get the suspicion that you (or certainly others reading this thread) will take this information to try to self-treat.

I've never heard of "late-onset" ADHD. Late-diagnosed, absolutely (I was diagnosed while I was in university), but to my knowledge there are no identified environmental causes that would cause ADHD to develop later in life. Professionals can help you determine whether you in fact have ADHD that was not diagnosed until now, or whether there are other factors.