"Adults with ADHD [attention-deficit hyperactivity disorder] have a 3.4-fold risk of developing dementia."
That was the conclusion reached by Nian-Sheng Tzeng and colleagues  who applied the 'big data' power of the Taiwanese National Health Insurance Research Database (NHIRD) to the question of whether yet more enhanced risk of adversity might be associated with a diagnosis of ADHD. Dementia by the way, reflects various symptoms/conditions pertinent to the decline of cognitive and other abilities.
Drawing on participant numbers in the hundreds, researchers determined that the risk of receipt of a dementia diagnosis in cases of adult ADHD was marginally higher compared with an asymptomatic (asymptomatic for ADHD) control group (5.4% vs. 4%). An association between ADHD diagnosis and risk of subsequent dementia remained after various, potentially interfering variables, were also taken into consideration ("age, gender, comorbidities, geographical area of residence, urbanization level of residence, and monthly income").
This is not the first time that enhanced risk of dementia in ADHD has been discussed in the peer-reviewed domain . As per that previous report from Golimstok and colleagues - "there is no clear explanation for the association found" - science still does not have many clues as to why ADHD might preferentially predispose to certain types of dementia. I say this on the basis that we don't really know that much about the biology of either condition/diagnosis. That's not however to say that science is not getting closer to some important clues, as the report from Zhang and colleagues  talking about an animal model of Alzheimer's disease that also had "a high frequency of antecedent ADHD symptoms" is detailed. More studies are indicated.
To close, a link to an article marking the recent passing of the father of the ADHD diagnosis: Keith Conners together with a warning...
 Tzeng NS. et al. Risk of Dementia in Adults With ADHD: A Nationwide, Population-Based Cohort Study in Taiwan. J Atten Disord. 2017 Jun 1:1087054717714057.
 Golimstok A. et al. Previous adult attention-deficit and hyperactivity disorder symptoms and risk of dementia with Lewy bodies: a case-control study. Eur J Neurol. 2011 Jan;18(1):78-84.
 Zhang Q. et al. Alzheimer's Model Develops Early ADHD Syndrome. J Neurol Neurophysiol. 2015;6(6):1-6.