In an earlier post on this thread I mentioned.
"When my parents studied and baptised the elder who studied with them said (according to my brother) that it was impossible for him to die of old age. This was due to faith, faith in the good news I believe."
I think that these two quotes from the Watchtower may show why he felt it was impossible for him to die of old age.
Watchtower July 15th 1961 page 423
‘And since faith is also “the assured expectation of things hoped for,” it follows that we must also have knowledge of what are the things we may and should hope for. Again, God’s Word provides us with the needed knowledge. It assures us that paradisiac conditions will be restored to this earth, and that on a world-wide scale, and that this restoration work can be expected to begin within the present generation.’
Watchtower September 1st 1969 page 532.
'Jehovah has made many promises of things yet to occur, One may hope that such things will come to pass, but such expression may only be a whimsical wish, not real faith. Faith is more than just an expectation. It is expectation that is assured to such a degree that it is able to support , uphold and guarantee that what is hoped for will become a reality. This is the meaning of hypostasis, a word common in ancient papyrus business documents in reference to guarantees placed on the transactions.
...Substantially the words [hy.po’sta.sis and e’leg.khos] mean that faith gives to things future, which as yet are only hoped for, all the reality of actual present existence,; and irresistibly convinces us of the reality of things unseen and brings us into their presence.” The Expositor’s Greek Testament, Vol.IV page 352.
That's why when 1975 came and went I still believed the end was close and stayed a JW and wasn't 'heartbroken, shaken, upset and finally depressed.' However I do feel it had some effect for years after