Question

In the quote, "Old age should burn and rave at close of day" is the use of "old age" instead of "old people" an example of synecdoche, or is it something else?

Answer 1

Yes - it can be said that "Old age" is part of "people who are advanced in years".

Explanation:

The quote is an excerpt from Dylan Thomas' Do Not Go Gentle Into That Goodnight.

The short answer is it can be an example of synecdoche (which is when you use a part of a name to represent the whole). In this case it can be said that "Old age" is part of "people who are advanced in years".

http://literarydevices.net/synecdoche/

And now to the far more interesting thing - the poem itself. Here's the full text:

Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.

Though wise men at their end know dark is right,
Because their words had forked no lightning they
Do not go gentle into that good night.

Good men, the last wave by, crying how bright
Their frail deeds might have danced in a green bay,
Rage, rage against the dying of the light.

Wild men who caught and sang the sun in flight,
And learn, too late, they grieved it on its way,
Do not go gentle into that good night.

Grave men, near death, who see with blinding sight
Blind eyes could blaze like meteors and be gay,
Rage, rage against the dying of the light.

And you, my father, there on the sad height,
Curse, bless, me now with your fierce tears, I pray.
Do not go gentle into that good night.
Rage, rage against the dying of the light.

https://www.quora.com/Whats-the-meaning-of-the-poem-Do-Not-Go-Gentle-Into-That-Good-Night-1

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Note how Thomas essentially writes about all sorts of people, all of whom are facing hard times, perhaps even death, and entreats them to fight for life, to not give up and roll over and accept death or hard times as a certainty. He acknowledges that the struggle itself may be hard and cause suffering but that the suffering is worth it.

The poem itself was written by Thomas to (or perhaps for) his father who was going blind.