How can I talk about theme of old age in King Lear as a main idea in the play?
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.