What is the tragedy of old age?

King Lear is about the universal tragedy of old age. This theme shows that Shakespeare had a broad world vision (Weltanschauung), as evidenced in that famous soliloquy in As You Like It where Jacques says, "All the world's a stage, and all the men and women merely players." Growing old is a tragedy in itself—a tragedy for every man and woman who lives long enough to have to experience it. Lear and Gloucester both suffer through the tragedy of being old, despised, unnecessary, and unwanted. The fact that both find themselves homeless in the wilderness is symbolic of the cold and lonely condition of old age. They have both been displaced by the younger generation. They both had their "entrances" and are about to make their "exits" from the world stage. They have lost their property and their rights. Somebody has to take care of them, as if they are children. Gloucester has even lost his sight. They both understand life as they never understood it before. Schopenhauer writes,
Only the man who attains old age acquires a complete and consistent mental picture of life; for he views it in its entirety and its natural course, yet in particular he sees it not merely from the point of entry, as do others, but also from that of departure. In this way, he fully perceives especially its utter vanity, whereas others are still always involved in the erroneous idea that everything may come right in the end.
Both Lear and Gloucester comment on the "utter vanity" of life. It is significant that the two old men in the play were once rich and powerful, surrounded by people and highly esteemed. Lear was actually the king, and Gloucester was an earl. They learn the bitter truth about old age is that they will soon have to die and will be deprived of everything, including their bodies. Nobody really cares about old people. They are only in the way. Younger people find them boring and annoying.
In Act II, Scene 4, Lear tells the plain truth to Regan when he says,
Dear daughter, I confess that I am old;
Age is unnecessary