That's just it, it's not just a "delay." Everything I have seen suggests that the brain literally functions in a measurably different way, and many of these differences are essentially permanent*. You're not going to wake up tomorrow left brained if you're right brained. (Not as a normal part of development, certainly there are traumas that can affect laterality).
This is one of the first studies to examine the structural brain anatomy and connectivity associated with an ADHD diagnosis and child as well as adult ADHD symptoms in young adults. It was hypothesized that an adult ADHD diagnosis and in particular ...
www.ncbi.nlm.nih.gov
Just something I pulled off a search. If you go looking for evidence of structural or functional changes in the brain associated with ADHD, with persistence in some individuals into adulthood, you will not be disappointed. Same with symptom persistence, measurable affects on neuropsych testing or other validated measures. It's something that can be easily observed in some people.
It's totally valid to debate what this means practically for treatment. But sort of waving your hand that all ADHD brain functions and affects on life in adulthood sort of vanish with age... it's not just an interesting medical hypothesis. It's just plain wrong and really flies in the face of a whole body of research and understanding. That's all.
Keep in mind, I'm not claiming ALL cases of childhood ADHD persist indefinitely. But at any age there will be some individuals persisting and some not.
*Yes, you can see adaptations and coping skills that render some of the persisting changes a
effectively symptom-less. And yes, not ALL the changes seen are permanent. But many are.