Here are some examples: *An* age: "The Cathedral will be finished in an age" This is similar to "Dinner will be ready in an hour" "The comet comes around once in an age" This is similar to "The milkman comes around once a week" The age: "This fish lived in the age of the dinosaurs" This is similar to "My party was on Saturday last week" "This fashion was popular in the age of discovery" This is similar to "This fashion was popular in the 1970's" 'in an age' means that 'age' is any* long time. For example, if something takes an age to happen, that means it takes a long time. It does not mean it happens in any *particular age.
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