The significance of old age and death in King Lear represent inevitable lessons that must be learned.
While King Lear is foolish in many regards, he speaks words in the opening of the drama that reflect understanding about old age and death. In Act I, scene 1, Lear describes the inevitability of both:
and 'tis our fast intent
To shake all cares and business from our age;
Conferring them on younger strengths, while we
Unburthen'd crawl toward death.
This understanding forms the basis of his story. The images that Lear uses to describe old age and death are significant. Human beings become an infant whose "crawl" as they age and leads to a final end of death. It is interesting to see Shakespeare open his drama with the main character articulating the significance of old age and death. Lear's words reflect that both cannot be escaped as they define human identity.
The significance of old age and death are the lessons that Lear must learn. He ends up having to prove the wisdom in the words that he speaks. Lear learns that all human beings must endure old age. It makes Lear a "shadow" of what he once was, humbling him. His "reservation of an hundred knights" is removed. His title and ownings are stripped. He literally loses the clothes he wears, and in the process, experiences his own "Unburthen'd crawl toward death."
Old age and death are inescapable. Shakespeare shows that old age is significant because we do not experience better things as we get older. Rather, we endure the ravages of age. King Lear's wife does not have to experience this because she died. Given the lessons that old age teaches King Lear, her end seems much more merciful than what he experiences.
When Lear asks "Is this the promis'd end," it is an acknowledgement that death is inevitable. As a result of old age and staring into the face of death, he has learned that his own actions triggered unbelievable sadness. Old age and death are the ultimate teachers which cannot be averted. The only respite is in individuals recognizing through actions and behavior that the "promis'd end" of old age and death awaits everyone.