I think they're referring to the tendency of the passage of time to accelerate as we age. Older people often tell me this, and as I get older time does seem to pass faster. So now that they have been married for over 40 years, and must be 60-something, 50 years seems like a shorter time than it did when they were young.

I suppose that the writer is saying that when you have experienced something for 50 years, 50 years doesn't seem so long.

the entire paragraph, please

Do you want to know what the entire paragraph means or are there specific words you don't know?