1. It is the workers who haven't had a holiday for ages.
2. It is ages since the workers had a holiday.
Are the sentences grammatically correct?
Yes, they are both correct. (2) is a plain statement. (1) needs additional context; it contrasts the workers with some other person or group of people who have been mentioned.
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Yes, they are both correct.
(2) is a plain statement. (1) needs additional context; it contrasts the workers with some other person or group of people who have been mentioned. E.g.:
A: I think the manager needs a holiday.
B: The manager?? It's the workers who haven't had a holiday for ages.