Then tell them about ADHD. The main problem is that most people have no idea what it is, or how effective treatment can be. ADHD is one of the most poorly named and broadly misunderstood conditions ever.

Or, when the context is right, just ask, "hey, did you ever get diagnosed as ADD?"

I once had a close friend ask if I had [other syndrome], and my answer was "I have all the symptoms but don't care to seek a diagnosis."

It was a very polite way to inform me, and a polite way for me to say that I was happy living my life the way I want to live it.