Autism is a specifically diagnosable disease. It first long before a child's age reason. It is highlighted by a disconnection from society and surroundings, but it also often brings with it other “skills” that non-autistic children cannot learn or develop.
Examples are the pianist who can play a 30 piano piece with no music sheets (he is blind) after hearing it once. The young man who can remember the models, colors and license plate numbers of the last 1000 cars passing him on the highway, or the various examples of talents of autistic children shown in “Rainman.” The movie was fictional but the abilities of autistics were not.
On the other hand, ADD or now often called ADHD is all too often a case of lack or imposed parental/adult controls. Children learn that they can get away with anything, and then do.
Example: a flight from Orlando to New York. An eleven year old boy would not stop running up and down the aisle, shouting, cursing, hitting people, etc. He ignored the seat bely sign and the stewardess’ semi-polite instructions repeatedly. The father's constant (whining )response was “What can I dooooo?”
Finally a man across the aisle from my wife gently grabbed the kid as he ran by. When the kid tried to squirm away, the gentleman discretely tightened his grip and whispered to the kid, “Listen you little brat, sit down, shut up, or I'm going to rip off your head and s**t down your throat.”
The kid sat down and was quiet the rest of the flight.