How about this sentence? (By 1996, the United States population exceeded...)

[Chiaki]

Where is the mistake?

By 1996, the United States population exceeded 261 million, with a median age of thirty-four making it one of the oldest national population on Earth.

[Yankee]

Hi Chiaki

The word 'population' is wrong. It should be plural (populations).
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[Conchita]

You must have meant only the word 'population' at the end of the sentence, Amy: one of the oldest national populations on Earth.

[Yankee]

.
Thanks for that clarification, Conchita.
You're exactly right.
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[Chiaki]

Thank you, Yankee and Conchita. Sometimes I become "blind", that was a simple mistake.