This should be correct:
It's ages since I last ate in restaurant
But can I say:
It's (has) been ages since I last ate in a restaurant
This should be correct:
It's ages since I last ate in restaurant
But can I say:
It's (has) been ages since I last ate in a restaurant
The most natural sounding phrase to me (AmE) is:
It's been ages since I last ate in a restaurant.
Since that is short for:
It has been ages since I last ate in a restaurant.