suffering. 

It is interesting that you have selected this as a main theme. Whilst, undoubtedly, Lear's advanced age is refered to and a part of the play, I am unsure whether it would classify as a major theme. I think we can link it in to the way in which Lear's experiences teach him that he is not superman, and that he is just a mere, frail mortal, a "mortal worm," as he calls himself. Certainly we are presented with an arrogant King at the beginning of the play who seems to have little awareness of the consequences of his actions. His desire to receive words of praise and love and the way that he ignores the reality of love rather than its pretence in his daughters shows that he is a character who definitely needs to understand something of the reality of his situation.

However through the brute force of the storm and the tragic events that occur in the final act, Lear is forced to admit his frailty and in addition the way that old age forms a part of his new humble condition. Note what he says in the final scene when he describes how he killed the slave that hung his daughter:

I have seen the day, with my good biting falchion
I would have made them skip. I am old now,
And these same crosses spoil me.

We see juxtaposed in this quote Lear's memories of his glory days when he had his full strength and then the reality of his present weakness, the "same crosses" refering to how adversities have taken away his strength. Our abiding image of the play after the tragedy of Act V is Lear as an old, blind man, clinging on to his daughter in her death, recognising that he is but a mere mortal in the cosmos and weak, frail and insignificant as a result. Certainly old age ties into this theme, but I would argue it is not a major theme in itself.

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.

They say there's no fool like an old fool, and King Lear certainly falls into that category. His advanced age appears to have played no small part in his fatefully foolish decision to divide his kingdom among his daughters. On the face of it, it is difficult to make sense of Lear's decision as anything other than an old man's whim. It's not even the case that Lear wants to retire and enjoy his twilight years free from the cares and worries of kingship. On the contrary, he still demands to be treated exactly the same way as he's always been, even though he no longer has a kingdom. In short, Lear wants to enjoy all the trappings of kingship without the burden of kingly responsibility.

Although Lear's decision to divide his kingdom does seem to be more than a little mad, it's only much later on in the play, when his entire subjective worldview has collapsed before his very eyes, that Lear actually does go insane. This is largely because Lear has been unable to construct an alternative worldview to the one that he himself destroyed. As such, he's unable to cope with the sudden reversal of fortune that his own foolishness has imposed upon him.

It says something about the chaos that Lear's parlous mental state has unleashed that Edgar has to disguise himself as a beggar called Tom O'Bedlam—named after a famous madhouse in London—in order to maintain body and soul in what was once such a well-ordered kingdom. Lear's madness doesn't just affect him personally: it has damaging repercussions for the entire realm and everyone in it.

As for death, this is a common feature of the tragic world that Lear has created. What's more, death, when it comes, is invariably brutal and senseless. There's no rhyme or reason to death in Lear's world, no sense of justice being done or divine punishment being meted out. In such a bleak, heartless environment, death almost seems to come as a blessed relief. Life itself has had no meaning ever since Lear turned the values of his society upside down. It becomes necessary, then, for death to fill the resultant void, providing at least some sense of stability in a kingdom that's rapidly falling apart at the seams.

Shakespeare did not get his greatest themes from ancient Greece or from anywhere but real life. King Lear is a tragedy about old age. Growing old is a tragedy in itself--a tragedy for every man and woman who lives long enough to have to endure it. Lear and Gloucester both experience the tragedy of being old, despised, unnecessary and unwanted. The fact that both find themselves homeless in the wilderness is symbolic of the condition of old age. They have both been displaced by the younger generation. They have lost their property and their rights. 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.

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 that the bitter truth about old age is that they will soon have to die and will be deprived of everything, even including their bodies. Nobody really cares about old people. They are only in the way. Younger people find them boring and annoying. Hamlet’s attitude towards Polonius is a good example. In Shakespeare’s Measure for Measure, the disguised Duke Vincentio tells the prisoner Claudio who is sentenced to death:

If thou art rich, thou'rt poor; For, like an ass whose back with ingots bows,

Thou bear'st thy heavy riches but a journey,

And Death unloads thee. Friend hast thou none,

For thine own bowels, which do call thee sire,

The mere effusion of thy proper loins,

Do curse the gout, serpigo, and the rheum,

For ending thee no sooner. Thou hast nor youth nor age, But, as it were, an after-dinner's sleep,

Dreaming on both; for all thy blessed youth

Becomes as aged, and doth beg the alms

Of palsied Eld: and when thou art old and rich,

Thou hast neither heat, affection, limb, nor beauty,

To make thy riches pleasant.

(Act 3, Scene 1)

Allow me to give you some pointers to help you think through your approach to answering this excellent question. It appears that, out of all of his plays, Shakespeare reserves this play for his most uncompromising presentation of old age through the character of the aging Lear and the realities that he has to face. Interestingly, so much of the wrath and anger that Lear displays in the play is actually the result of his own stupidity and his desire to prize appearance over reality. This of course operates in many different senses: he prizes the insincere declaration of love from Regan and Goneril over the reality of the love that Cordelia has for him. Likewise he wants to keep the appearance of the crown, whilst not having to deal with the reality of the day to day responsibilities of being King. Therefore, although he initially expresses anger against his daughters when they expel him from their household, he comes to realise that this anger must be in part directed at himself. Note how he begins by blaming others for his situation in Act II scene 4:

If it be you that stirs these daughters' hearts
Against their father, fool me not so much
To bear it tamely; touch me with noble anger...

Here he swears revenge against Regan and Goneril for their treatment of him. However, at the end of the play, he comes to show a much more accepting position when thinking of the old age that allows him to be mistreated and makes him weak:

I have seen the day, with my good biting falchion
I would have made them skip. I am old now,
And these same crosses spoil me.

Here we can see a move away from blaming others to an acceptance of his position and the necessary physical weakness that comes with old age. Thus, although we see anger and wrath expressed through Lear's character, as the play and Lear himself develop we see Lear accepting more the realities of old age.