This isn't really part of our bodies getting old but something that a chosen few have to deal with.
As the 1990s ended, my grandmother was diagnosed with Alzheimer's. As my cousin said in a poem, it seemed like she raised 100 kids, and that's not much of an exaggeration. She helped my mother raise me, so I guess I was actually the last of that long line. It was difficult to see her fade away, eventually getting to the point that she no longer knew who I was. She died in 2005 and we looked at it as a blessing. My grandmother had been gone for years, it was just her body that finally let go.
Now we are nearing the end of another decade and I find myself in the same position. My mother was recently diagnosed with the same disease. She still recognizes me, even though she can't always come up with my name. She sometimes gets angry for no apparent reason and sometimes repeats the same garbled rants about "Taking this up here and going down there", no matter what the subject is. It's the first steps on what I already know is a long road because I've walked down it before.