Dotage

A person's dotage is the period of their lives when they are on the downward path into senility.

I can often be heard to claim that I dislike euphemisms.  This is a lie, in part.  What I dislike are bad euphemisms.  Dotage, on the other hand, is a very good one.  It refers to something that is sad at best and tragic at worst.  It is a horrible thing to watch a vibrant mind decay.  When you say that a person is in their dotage, though, it makes it sound like they're just old and silly and fun and enjoying themselves and their grandchildren, possibly playing in a greenhouse somewhere (oh, and they're British, for some reason).  To say that someone is in their dotage makes it sound somehow like they're vaguely high all the time, rather than in the grips of a terrible disease.  That, my friends, is a good euphemism.