It does tend have a stigma attached, but having an accurate diagnosis can be a real help to a child with a SEN.
It can significantly help them at school, and not just academically but in terms of self esteem, confidence and learning their own learning strategies that'll set them up for life.
It can help you realise your child isn't doing things to wind you up, but simply can't remember those two hundred times you've already asked them put your shoes on please, put your shoes on please, put your shoes on please... It can help you help them by understanding what makes them tick and what is counter-productive.
As far as the 7 years old mark goes, the only time I've seen it referenced is that traits should be apparent before this age for it to be diagnosed.
Just a thought too, if you think your DC may possibly have ADHD, it might be worth acquainting yourself with dyspraxia and austism spectrum (aspergers) as there can be overlaps in traits and diagnoses - our DS is mainly ADHD with the odd trait that falls elsewhere.