I agree that adult day care exists and is for people with (early / mild to moderate) dementia. However, in my experience, it is almost nonexistent in the US on a daily 8-to-5 basis like child day care. Caregivers who are lucky might find in their community one or two volunteer-run church-based programs, each lasting for about two hours a week, which is enough time to go to the grocery and do another errand or two. Other than that, the choice is usually to pay for a home aide, or pay to leave the person at a residential memory care facility during the day, just not have them spend the night. Either or both of those can be helpful in the short to middle term.
For what it's worth, even knowing about adult day care for dementia, my first impression, like Norbert's and dude's, was that Corker was probably just referring more to childish behavior and pettiness, the tit-for-tat, me-first mentality that most people grow out of in kindergarten but that Trump and his advisers such as Bannon still display. It reminds me of the book 'All I Really Need to Know I Learned in Kindergarten':
1. Share everything.
2. Play fair.
3. Don't hit people.
4. Put things back where you found them.
5. CLEAN UP YOUR OWN MESS.
6. Don't take things that aren't yours.
7. Say you're SORRY when you HURT somebody.
Which I've always thought was a pretty fatuous title for adults; but on the other hand, when you read about not only Trump and his cronies, but also families like that of the Las Vegas shooter, you wonder how some people were brought up.
I also agree that if he really couldn't remember the word 'ocean' or other descriptive words, it could easily be a sign of mild cognitive impairment.
But his use of simple language (3rd-grade reading level or whatever) for his speeches could also be a choice based on his target audience -- people who don't trust elites who use big words. And as tigger says, forgetfulness and lack of concentration could also easily be a sign of stress, and/ or normal aging.
So I don't think anyone can or should assume dementia just from his public appearances. As Reagan showed, and as we have found with relatives who eventually developed dementia, it's easy to look back and apply the eventual diagnosis in hindsight, but much harder to analyze someone's behavior as it happens, gradually.
The other aspect of Trump's behavior that does give me pause is his anger. That could indeed be related to some loss of cognitive function -- but again, when someone already has that type of personality trait and loses inhibitions only gradually, it's very hard to know what to ascribe to a disease process and what's just 'the way he is.'
The one thing that makes me hopeful that Mr. Trump may have more self-control than he appears to -- well, apart from the group of advisers, who I hope can help remind him how to do the adult thing -- is the report that he is not a drinker. If he were consuming alcohol at the same time as confronting some loss of cognitive ability, that would indeed be more worrying, and harder for those who love him to deal with.