But rather than thinking that if you have ADHD you could be more likely to get Alzheimer, I think based on this study, it would be better to think, that there's something that can possibly cause all of those 3 more likely to happen.

What I'm trying to say, is that there's a gene X, that could mess with your ability to pay attention, decrease your IQ and increase risk of Alzheimer's.

You can still have ADHD, but it doesn't mean you immediately have higher risk of Alzheimer's.

To me it seems more likely that there's just this one thing that can cause issues in all sorts of different mental faculties.

Like in an extreme case if we cut your head off, you would have 0 ability to pay attention, 0 ability to have a decent IQ and you would have 0 ability to memorise things. It could just be something that impacts everything on a less milder manner. And it's a bit misleading to start the title with ADHD