I wouldn't be surprised if 15% of the population had ADHS. I'm also convinced it's not really a disorder, but a different writing in the brain that is (or was, in prehistoric times) beneficial in a lot of ways.
People with ADHS can achieve an impressive level of focus, determination and endurance if an activity interests them, can find creative solutions to problems because they think differently, and function better under a certain type of stress.
IMO they'd make great stone age hunters.
The problem is, our modern society doesn't fit their mindset at all, so they suffer in it and need medication and therapy to function in today's world.