If you are doing the same or broadly similar work ('like work') as a worker of the opposite sex in the same establishment, or at another establishment where common terms and conditions apply, you are entitled to equal pay. Normally, there has to be someone of the opposite sex in the same employment (a 'comparator') with whom you can compare your work.
Where a woman has evidence of direct sex discrimination in her pay but there is no actual comparator doing equal work, it can be possible to claim sex discrimination based on a hypothetical comparator. An example of direct sex discrimination on pay would be an employer who tells a female employee she would get paid more if she were a man, but there are no men employed by her employer.
Differences such as payment for extra responsibilities or payment based on different work locations are permitted, providing the differences are not related to the sex of the worker.
The law applies to all workers, regardless of age.