I am unsure how these matters function in the UK, but I can shed some light on why "20 years". Note that I am a clinical psychologist and specialise in ADHD, as well as having grown up as an ADHD kid (I recognised this when I was 40. I am now 74).
ADHD was considered a diagnosable only in children until 2013, when DSM V (universal diagnostic manual) was published. This opened diagnoses up to adults as well. I have been working in this area for about 30 years.
What needs to be stated at the outset is that (1) there is a spectrum for severity, from mildly affected to severe. The prevalence is around 10%, but there may be many undiagnosed at the mild end of the spectrum, who only become overt when there are life challenges. (2) While most people believe that ADHD (previously known as ADD) is about Inattention and Hyperactivity, that is an increasingly outdated perspective. It is really about disorganisation and related emotional dysregulation (or emotional sensitivity).
Diagnosis should be broad and aimed at excluding other look-a-likes. There are many reasons for the symptoms and not all lead to ADHD.
Medication is not needed except for the more extreme end of the spectrum, and even so psychological methods are important - pills do not develop skills. I have never had ADHD medication. I emphasise this as there has been a world-wide shortage of Vyvanse, the most popular ADHD medication for adults, and many were in a state of panic that their lives were going to end. That is a total overreaction. Find a clinical psychologist (called consulting psychologists in the UK) who specialises in ADHD.
Regards from Perth
Derek