Some Americans say they don’t understand how former President Joseph R. Biden Jr. could have only recently learned that he had an aggressive form of prostate cancer that had already spread to his bones. How could the former commander in chief, a man with access to high-quality medical care, not have known earlier that he had such a serious condition?

Mr. Biden, 82, has not disclosed details about when the cancer was discovered, or whether he had been regularly examined for prostate cancer. It may be that he had not been screened. Guidelines from professional organizations that advise doctors and public health officials recommend against screening for men over age 70.

But many men, in consultation with their doctors, continue screening into their 70s, which is not unreasonable if the man is healthy and has a life expectancy of at least 10 years, said Dr. Scott Eggener, a prostate cancer specialist at the University of Chicago.

Prostate cancer experts also say, though, that even if Mr. Biden had been screened regularly, it’s entirely possible the cancer was not detected till recently. They said that some men suddenly find out they have advanced prostate cancer even after being screened regularly year after year and told they have a clean bill of health.

It is unusual, but it does happen.

“I have an entire collection of what I call rocket PSAs,” said Dr. Ian Thompson, a prostate cancer specialist at the University of Texas Health Science Center in San Antonio. These are men, he said, who are screened year after year with the PSA, a blood test that can pick up signs of prostate cancer. Year after year, their PSA is very low. Then, suddenly, it soars.

He also sees men with advanced prostate cancer who have normal results on their PSA screening tests.