This is a longish letter from Seneca, but he eventually makes some excellent points. He starts off by talking about how language and words have changed and evolved over time before getting into Plato's thoughts about ideas and forms and what exists or not. It can be a bit opaque to read, if you don't have some background of what Plato is talking about. I've done some reading on this and reading through the first half of Seneca's letter was a bit of a struggle. If you need a brief summary of Plato's theory of forms, see this link.
Beginning around verse 25 of the letter, Seneca writes,
"Very well," say you, "what good shall I get from all this fine reasoning?" None, if you wish me to answer your question. Nevertheless, just as an engraver rests his eyes when they have long been under a strain and are weary, and calls them from their work, and "feasts" them, as the saying is; so we at times should slacken our minds and refresh them with some sort of entertainment. But let even your entertainment be work.
In sum, Seneca went through all this as a diversion to 'slacken [his] mind' for entertainment! After this verse, is when the letter gets better, in my opinion.
He explains why he did the analysis of the essence of things.
I try to extract and render useful some element from every field of thought, no matter how far removed it may be from philosophy. Now what could be less likely to reform character than the subjects which we have been discussing? And how can I be made a better man by the "ideas" of Plato? What can I draw from them that will put a check on my appetites? Perhaps the very thought, that all these things which minister to our senses, which arouse and excite us, are by Plato denied a place among the things that really exist. Such things are therefore imaginary, and though they for the moment present a certain external appearance, yet they are in no case permanent or substantial; none the less, we crave them as if they were always to exist, or as if we were always to possess them (emphasis added).
What Seneca gets out of that analysis is that the very things which we think are real and exist, in fact are imaginary. Therefore, while a Stoic may not agree or adhere to the Platonic ideas of Forms, we nonetheless can see a similarity in how we view indifferents and things that cause us to desire and crave. Seneca applies the discipline of assent in seeing things as they really are:
We are weak, watery beings standing in the midst of unrealities; therefore let us turn our minds to the things that are everlasting. Let us look up to the ideal outlines of all things
I understand this to mean that we Stoics recognize that there is a sole good and on it we should rest our desires - namely ageless and timeless ideas such as moral virtue. Seneca doesn't quite come out and explicitly state this in the letter, but he tends to focus more on the "defects of the body" and our impulses and "pleasures" and how we should "acquire the ability to control and check those pleasures" which ail the rest of humankind. He advocates that we should mimic Socrates who practiced "frugal living, by setting a limit upon all that rouses the appetites, and by painstaking attention to himself."
Next he gets into his opinions about old age and when one should consider enduring old age or ending his life.
The question, therefore, on which we have to record our judgment is, whether one should shrink from extreme old age and should hasten the end artificially, instead of waiting for it to come.
It all depends on the circumstances and Seneca wants to do the right thing for the right reasons. He writes his thoughts on the matter of extreme old age and what he thinks is the wise course of action.
I shall not abandon old age, if old age preserves me intact for myself, and intact as regards the better part of myself; but if old age begins to shatter my mind, and to pull its various faculties to pieces, if it leaves me, not life, but only the breath of life, I shall rush out of a house that is crumbling and tottering.
Here he makes it clear - as long as I have my wits about me, I'll endure living. But if I lose my wits, I'm outta here!
I shall not avoid illness by seeking death, as long as the illness is curable and does not impede my soul. I shall not lay violent hands upon myself just because I am in pain; for death under such circumstances is defeat.
I won't let myself off easy by killing myself to avoid the pain from a curable ailment.
But if I find out that the pain must always be endured, I shall depart, not because of the pain but because it will be a hindrance to me as regards all my reasons for living. He who dies just because he is in pain is a weakling, a coward; but he who lives merely to brave out this pain, is a fool (emphasis added).
But if he is going to endure pain that will never leave, he finds justification for ending his life. If the pain hinders him for his reason to live, then he will depart. We won't be a fool to bravely live out in pain. He looks to his reason to live and if the pain prevents that, then he will go.
In sum, he will endure extreme old age, as long as he has his cognitive abilities and the pain does not prevent him from accomplishing his own reason or reasons to live (i.e. perhaps living to talk with loved ones, or writing letters to friends or something else).