Here are two paragraphs from the very end of the first chapter --"Lost Gods"-- of John Buchan's The Island of Sheep, published in 1936, that I'm finding resonant. Richard Hannay, of The Thirty-Nine Steps, is our narrator, and he's reached his fifties . . . .
I continued my journey -- I was going down t the Solent to see about laying up my boat, for I had lately taken to a mild sort of yachting -- in an odd frame of mind. I experienced what was rare with me -- a considerable dissatisfaction with life. Lombard had been absorbed into the great, solid, complacent middle class which he had once despised, and was apparently happy with it. The man whom I had thought of as a young eagle was content to be a barndoor fowl. Well, if he was satisfied, it was no business of mine, but I had a dreary sense of the fragility of hopes and dreams.
It was about myself that I felt most dismally. Lombard's youth had gone, but so had my own. Lombard was settled like Moab on his lees, but so was I. We all make pictures of ourselves that we try to live up to, and mine had always been of somebody hard and taut who could preserve to the last day of life a decent vigour of spirit. Well, I had kept my body in fair training by exercise, but I realized that my soul was in danger of fatty degeneration. I was too comfortable. I had all the blessings a man can have, but I wasn't earning them. I tried to tell myself that I deserved a little peace and quiet, but I got no good from that reflection, for it meant that I had accepted old age. What were my hobbies and my easy days but the consolations of senility? I looked at my face in the mirror in the carriage back, and it disgusted me, for it reminded me of my recent companions who had pattered about golf. Then I became angry with myself. 'You are a fool,' I said. 'You are becoming soft and elderly, which is the law of life, and you haven't the grit to grow old cheerfully.' That put a stopper on my complaints, but it left me dejected and only half convinced.
--John Buchan, The Island of Sheep