Women should be doing this before 21 years. My sister was dx with cervical cancer at 18, she kept putting off treatment, in denial I think, and when she got treatment the cancer had spread so badly, she had tons of chemo and radiation until there was no more they could do for her. In the end she recovered but it was very close.
My mother also had cervical cancer when she was 34, she had a full hystorectomy, as a result.
I would not wait until 21 , my sister would be dead if she waited till then. They do recomend 21 or within three years of becoming sexually active. I don't see any reason why not to get checked sooner than 21, especially if you have a family history.