In general during diagnosis (I can attest to Germany), it must be credibly shown that multiple symptopms have been present both during child as well as adult age.
Typical patterns are several aborted apprenticeships or studies, inability to hold steady employment over several years, problems with romantic relationships or long lasting friendships.
The tl;dr is that we think ADHD traits had positive selection pressure for 90% of human history, which is why ADHD is so common. ADHD is actually believed to be dramatically under-diagnosed in certain population slices, like adult women, because the diagnostic criteria was developed based on predominantly male/child subjects.
In the context of scouting for a nomadic hunter-gatherer tribe, ADHD traits seem like a gift.
In the context of modern American society, ADHD traits can lead to strong negative experiences spanning education, career, and relationships. In some contexts (like entrepreneurship), ADHD traits can still be a gift - but I'd hazard a guess that percentage-wise, a larger % of the ADHD population winds up in prison vs. becoming an entrepreneur.
Similar to autism, ADHD is a spectrum neurological disorder. "Disease" is the wrong term to use; "disorder" is the correct medical term. ADHD traits become a disorder they have a negative impact on development (childhood) or negative impact on well-being as an adult.