The criteria in the EU are also much more strict (and "outdated") because we generally use ICD-10 still, whereas the US used the DSM.
Thats true. Also there must be documented start in childhood.
Depends. I was diagnosed independently in EU (Poland) and in US (California) at the age of 32. They didn't need a paper documentation, just asked about my childhood.
Ok. In Sweden I had a lot of trouble since my parents refused to cooperate (friends of scientologists, don't 'believe in diagnoses'), but I eventually got my living grandparent to help as much as he could after about a year.
I got suggested to get tested after failing out for the second time and the testing showed very much lower than average sustained attention. I never myself thought about getting tested since I did really well at a lot of tasks and just couldn't figure out how to micromanage all the weird administrative tasks of every day life. If I didn't have to do those everything would be going amazing, but instead it's sunk me into (small) debt and a period of homelessness a few years back.
I think a lot of people in the EU (where I am) would have gotten diagnosed if tested, but we seldom test school age children who don't have a very hard time in school. If you do somewhat ok or manage it seems odds are you can go undiagnosed forever.