You have to have had childhood onset to have ADHD. If problems with attention emerge during adulthood, that might be the beginning of a psychotic disorder, in which case the priority should be to treat the underlying psychotic disorder.
Sure, but that doesn't mean childhood diagnosis, so I’m not sure how that's relevant to the discussion.
Source: I'm a doctor, also https://www.cdc.gov/ncbddd/adhd/diagnosis.html
Specifically, it’s important to differentiate between symptoms “emerging” in adulthood vs. being identified in adulthood. What they wrote implies that most adult diagnoses are likely not ADHD if they did not get a diagnosis earlier in life.