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:
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.