Sorry if I wasn't being clear, what I was trying to say was that young children (under the age of around 7) typically exhibit traits that are associated with ADHD, but they are normal traits for that age.
So they grow out of those traits (inattentiveness etc) but they wouldn't be diagnosed with ADHD at that age, because those traits are normal for that age group, therefore it's not ADHD. Just typical traits/behaviour for that age, which is why doctors wouldn't typically diagnose ADHD at that age.
If those traits are still present after that age, that's not considered usual for that age group and that's when people would tend to go down the diagnosis route.
Obviously I'm sure there would be the odd case who were diagnosed with ADHD and it turned out to be something else (an alternative diagnosis, or perhaps just a later -than-usual development) but I think it's rare.
So I'm trying to say that people don't grow out of ADHD, just traits that present in a similar way to ADHD, but that are exhibited by certain age groups.
The paediatrician explained ADHD to me when my son was diagnosed and it's to do with the rate that neurotransmitters in the brain regenerate (I think!) so it is a physiological thing.