A: I haven't seen you in ages.

How have you been?
B: I'm fine.
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Is this dialogue correct?
Do we have to answer,'I'm fine.' or 'I've been fine.'?
Can we say 'in an age' instead of 'in ages'?

The dialogue as you've written it is correct. Just idioms, that's all -- but those are the correct phrases.

=)

The dialogue mentioned is correct. Both responses, "I'm fine" and "I've been fine," are acceptable. However, "I'm fine" is a more common and informal response.

Regarding the phrase "in ages," it is commonly used instead of "in an age" to express a long period of time that has passed without seeing someone. So, saying "I haven't seen you in ages" is the appropriate phrase in this context.