noun
BAD : The history of the town dates back to the Middle Age.
GOOD : The history of the town dates back to the Middle Ages.
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middle age = the period in a person's life between youth and old age: 'People who live this type of life are lucky if they reach middle age.'
Middle Ages = the period in European history from about 1100 to 1500 AD: 'Life in the Middle Ages was very simple.'