The link between ADHD and Alzheimer’s is based on only one study that has structural flaws. Another study did not show any link between ADHD and Alzheimer’s. I think it is premature to assume that people with ADHD have an increased risk of dementia, without more research.
I am not an expert on the prevention or treatment of Alzheimer’s disease, but to reduce one’s chances of developing the condition, I would recommend a healthy eating plan (lots of whole foods, fruits, and vegetables, and staying away from processed foods), a balanced lifestyle (minimizing stress at the job and in your home life as much as possible), adequate exercise, and keeping the brain active and challenged.