Speaking as the mother of an ADHD (combined type) child I can tell you that it is not as easy to get a diagnosis as everyone seems to think it is. And, although I get the impression that some of you think that it is just an excuse for naughty children, it isn't (those "naughty" children usually have an additional diagnosis such as ODD or PDA). This is a condition that has a significant effect on children in many parts of their life and is lifelong.
I had a good inkling that DD (now 14) had it from the age of about 3 and exhibited traits in all areas of life from that age - not that I initially concluded it was ADHD (she is my only child so didn't really have a point of reference, but hindsight is 20/20). Initially, I had a demob happy (i.e. soon to retire) GP saying that because DD had sat on the floor and played with a toy in her office for a short period of time there was no way she had it and we were in and out in less than 5 minutes - she also said to go through the school if I was worried in future. Her primary school teacher (whom she ended up having for 3 years) downright bullied her (he was well known for playing favorites in his classes and DD was certainly not one of them - I used to have her classmates telling me stories of how he treated her in school - but he was seemingly untouchable* and there were no spaces in any other local school) and insisting that she was just a difficult child when I repeatedly asked if I/they should be arranging some form of assessment - note, I don't say naughty as she wasn't badly behaved, but instead incapable of staying in one place for long, impulsive, struggling to absorb instruction, in her own world etc.
I eventually had one of her primary school teachers approach me in year 6 and suggest she should be assessed - she was the only teacher at that school that had DDs back, and they managed the poor woman out because she was going to apply for the Head role and the acting Head (who had taught there for many years) wanted her out of the way so she could get it (this is common knowledge now). I was LIVID.
As soon as she started high school she was identified as requiring an assessment and the school and (new) GP were on the case, she had an assessment a few months later - we were lucky with the wait time, the waiting list to be assessed on the NHS can take years - they're usually carried out by either CAMHS or a community pediatrician who have a million and one other children to assess and treat for a variety of conditions, not just ADHD, so you can imagine their workload.
The assessment process is lengthy and multi-faceted and involves teachers as well as parents/carers. They used clinically recognised assessment tools and techniques and there are very strict diagnostic criteria.
I am on a couple of forums for parents of kids with ADHD, there are plenty of people out there whose children are still waiting to be seen after well over a year, many who have had an assessment and have been "rejected" (for want of a better term) after assessment and feel abandoned, many who didn't even get offered an assessment as their child didn't fit the criteria.
DD started medication about 5 weeks ago and I just had her first parent's evening. Although I suspect she needs to increase her dose slightly, most of her teachers have seen a marked improvement in her since starting. This is a hardcore stimulant drug, similar to an amphetamine, I have to show my ID to collect it from the pharmacy - it is not something that is doled out lightly.
Understanding of ADHD is still in its infancy, there is no blood test or scan that can diagnose it, but they know much more than they did, such as the part that neurons and their reuptake of certain neurotransmitters (e.g. dopamine) play, what causes ADHD such as genetic factors, as well environmental factors (exposure to certain chemicals in pregnancy or infancy) or alcohol/drug/tobacco use in pregnancy. So with an increase in understanding and identifying the condition, plus the prevalence of both environmental and social factors playing their part in children having the condition - THIS is the perfect storm.
The condition is something that has affected pretty much every part of DDs life, from schoolwork to friendships/relationships. Hopefully, it won't stop her from fulfilling her goals in life, but it is going to be a long hard slog for her (and us!) to get her there, hopefully the medication will help her achieve those dreams.