I happen to know multiple people who work with and around her. She is deeply senile. Her staff run the show. This is well-known in DC, including by Dems. It's not a partisan analysis.
Now, does that mean every 90 year old is like her? No, of course not. Some 90 year olds are very mentally sharp. But it's well known medically that, in general, we cognitively decline with age. I don't want an arbitrary age cap for public office, but at a certain age we should probably require mental competency tests. I don't know what the magic number should be.