I think that "in the past" our species didn't have to worry about "senility" (and what comes with it) because people were dying while their brains were still at full capacity/capability and it was only the body that was "failing".
The jump from the lifespan of 50 to 90 to just a few decades must have made a huge difference in the appearance of this "new problem" of having 3-4 generations being alive at the same time.
I remember a Great-grandparent was a very rare thing. Now with people living up to 100 and people having kids at an earlier age, we get to see it far more, and this will most likely continue.