They will refuse you before the age of 25 if you haven't had one previously.
It was that once you were sexually active, they would test you. But some years ago they put the age limit up to 25 as results are not as reliable as they should be due to the amount of unsettled hormones in women under the age of 25.
They will send you a letter when you are 24 for when you turn 25 or sometimes a few months before.
If you have a history of cervical cancer or have experienced severe symptoms, they will test you.