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.
Newcorn JH, Weiss M, Stein MA. The complexity of ADHD: diagnosis and treatment of the adult patient with comorbidities. CNS Spectr. 2007;12(suppl 12):1–14. quiz 15–16.
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.