The scientific, peer-reviewed answer is that it is significantly under-diagnosed in adults as well as in those AFAB of all ages. Most sources say up to 80% of adults with ADHD are undiagnosed and/or untreated.
I agree with you.
I think argueing if adhd might be over or under diagnosed makes adulds feel even more ashamed.
I also don’t like blaming self diagnosis. Women having a hard time finding professionell help, cause they never fit into stereotypical adhd behaivor. They seen as overreacting and emotional.