How can I talk about theme of wrath in old age in King Lear?

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:

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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:

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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.