If I had waited until age 25 to get screened for cervical cancer, I might very well be dead. I was diagnosed at age 22 with pre-cancerous cells (this was long before HPV vaccines were available to teens) and had to undergo several procedures and get re-tested every three months until the PAP results were clear again. To add insult to injury, my lousy insurance refused to cover any of it and I almost had to declare bankruptcy from all the medical bills. At the time, the screening guideline here in the US was once a year beginning at age 21 or whenever you become sexually active. If I had to give a recommendation to others, I would strongly advise getting tested as soon as you become sexually active. I found out the hard way that all it takes is one infected partner--in my case, it was my first and only partner at the time, and he had been given a full STD screening (I made sure he showed me the results before we started having sex) and I still ended up contracting HPV from him. Like I said, if I waited another four years to get tested, my cervical cancer might have been too advanced to treat.

TL; DR: the age to start getting screened for cervical cancer is the age when you become sexually active. Never mind the government guidelines.