How does King Lear portray the theme of "age?"

These words, spoken by Edmund just before he goes to betray his father to the Duke of Cornwall, may well be the most important in the play. These seven words sum up what the play is really about. Shakespeare is dramatizing a supreme fact of life--that each generation is followed by a generation which will inevitably assume all its power and property and will eventually bury or cremate it. Keats expresses the idea in his best poem, "Ode to a Nightingale," when he says to the bird, "No hungry generations tread thee down." Both Lear and Gloucester are being trodden down by members of the hungry generation which they themselves have helped to create. Goneril and Regan have acquired everything that belonged to their father King Lear. Edmund, the illegitimate son, is about to supplant his own father as the Earl of Gloucester. It is a simple, inescapable, inevitable fact of life. At any given time there are five generations in existence: the children from ages one to twenty; the youths from twenty to forty; the middle-aged from forty to sixty; the elders from sixty to eighty; and the decrepit and nearly invisible generation from eighty onwards, who are dependent and powerless. Those from sixty to eighty have been pushed aside by the ones from the really strong, mature, ambitious, and hungry generation of people forty to sixty, and these people will find themselves losing their vigor and self-assurance, wondering what's it all about, as they are being infiltrated by the generation from twenty to forty. Life is like a moving sidewalk conveying everyone towards the same destination. The fact that Lear has one kind daughter and Gloucester one kind son does not change their fathers' fates. Both old men die, and both are glad to do so.
Here is a pertinent quotation from Sister Carrie by Theodore Dreiser, a great American writer who deserves much more attention and respect than he currently receives:
A man’s fortune or material progress is very much the same as his bodily growth. Either he is growing stronger, healthier, wiser, as the youth approaching manhood, or he is growing weaker, older, less incisive mentally, as the man approaching old age. There are no other states.