Currently watching my 95 year old grandma slowly fade away mentally. Been tough as I see her a few times a week and she doesn't remember anything from my last visit, which sucks because I've got my first baby on the way and have to tell remind her everyday her she's going to have her 15th great grandchild. Happened rather quickly too as she was sharp only a year ago.
Lost my great grandma to Alzheimer's when I was around 10 and that was tough as a child having her not know who I was, couldn't imagine having it happen to my own parents.