Smear tests are a way of searching for abnormal cells in the cervix, which can develop into cervical cancer if left untreated.
The free tests are currently offered to women in England every three years, if they are aged between 25 and 49.
They used to be offered to women aged 20 and over, but the age was increased in 2003.
Women aged between 50 and 64 are screened every five years, and women over 64 are only tested if they have not been screened since they were 50 or if they have recently had abnormal test results.
This is because women aged under 25 or over 64 are deemed as 'less at risk' of developing the disease.
The Department of Health ruled in 2009 that the age limit for smear testing should stay at 25.
It was after calls for smear testing to start at 20 years old, to reflect the number of sexually active younger women.
Any women who are overdue a test can book one with their GP, or they may be offered one the next time they have a doctor's appointment.
Younger girls are now offered a vaccination against the human papilloma virus, which can cause cervical cancer.
The HPV vaccine is given to schoolgirls aged between 12 and 13 years old.