Question

Correct tense form
I (not eat) in a restaurant for ages.

Answer 1

I have not been eating in a restaurant for ages.

OR

I had not been eating in a restaurant for ages.

Answer 2

The correct tense form for this sentence can be either of the two statements.

I have not been eating in a restaurant for ages.

OR

I had not been eating in a restaurant for ages.

Both the sentences are correct and have the right tense to explain the same. So any of the two sentences can be considered for the same.