Hate when people think children, especially those with things like ADHD, cannot be responsible for their actions.
My ds has high functioning autism. He learns everything by being taught explicitly. He doesn't have an aggressive, bullying or malicious bone in his body. But he has in the past hit out because of communication difficulties, frustration, and looking for attention, as he didn't know how to communicate appropriately. We have now taught him (an ongoing process) how to communicate appropriately so he no longer hits etc.
If we hadn't taught him this, he might still at the age of 5 be hitting as it was his way of getting attention or getting what he wants.
Or if he'd had different parents or teachers, he might have learnt that hitting is ok.
Either way, he would not have been responsible in the way you mean, because he copies what he is taught, shown and told. He does not instinctively know the difference between 'right' and 'wrong' because these are on the whole social constructs, and conditions like ASD mean that there is a huge impairment in, or lack of understanding of social rules. He knows it is 'wrong' to hit because we have taught him this.
You really do have no idea about neurological/behavioural conditions. I understand you had a shit experience when you were six, but you have no idea what you are talking about.