What does this phrase mean: 'for ages'? Why don't we use an article here?

[robin]

Test No. incompl/inter-57 "A Night Out", question 2

Nothing unusual you might say but we haven't visited a cinema for ..........

(a) periods
(b) ages
(c) times
(d) lengths

Test No. incompl/inter-57 "A Night Out", answer 2

Nothing unusual you might say but we haven't visited a cinema for ages.

Correct answer: (b) ages
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why is there no article in front of ages?

[Alan]

No article is used because ages is not specified.
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