There is no single correct answer. The first consideration is family history.
If you have a family history of genital cancer then the examinations should start by age 25-30. This is particularly true if it is on both sides.
Genital cancer is defined as:
Female: Breast, Ovarian, Cervical, Uterine and defined as adenocarcinoma which is a hormone dependent tumor
Male: Male Breast, Testicular, Prostate, and defined as adenocarcinoma which is a hormone dependent tumor.
The closer the cancer gets by relative to you personally the more cautious you should be.
In the case of my Stepbrother his Father, Aunts and Uncles and several other relatives on that side all died of adenocarcinoma cancer. On his Father's side, the family history was for the record books with this variety not only in his Father but taking the lives of multiple aunts and uncles as well.
On his Mother's side there were three documented cases with the people all losing their lives to the disease process.
He started monitoring at age 25. He should have gone to six month screenings at age 40 because of the history.
Look at the numbers on this one and also remember that regular PSA tests are also an indicator of where you are headed. I have a PSA every couple of years. I am on male HRT and I also do it because of that. My PSA levels are extremely low and far below average for my age which considering the HRT is a great thing.
If there is a family history that is "bad news" every year from 25 on.
If there is a distant blood lone relative that is a single then every 2 years to age 40 with every year after that. If the family has an extensive adenocarcinoma history then every year from 25-40 and every six months after age 40.