i'll outline the current United States Preventative Services Task Force (USPSTF) guidelines for screening. it's a lot, but hopefully will help.
the USPSTF could not find evidence for, or against, the use of PSA and digital rectal exam (DRE) in the overal outcome of prostate cancer. they did however find good evidence that the screening tests (PSA & DRE) can aid in early detection. they conclude that if early detection improves the outcome, then the population what benfits the most will be men 50-70 in the average risk group, and men over 45 in the increased risk group. the increased risk group is made of African American men and those men with a positive family history (first degree relative with prostate CA). the finally conclude that men with multiple medical problems, or those with a life expectancy of less than 10 years, are unlikely to benefit from screening. remember that screening also carries it's own risks versus benefits (false positives, biopsy, complications, anxiety).
the USPSTF recommends against the use of screening tests for lung cancer.
the USPSTF recommends the use of mammography every 1-2 years in women 40 and over. This can be used in conjunction with clinical breast exam, although this is not a requirement. The first mammogram may be done at 35 in women that are symptomatic or high a positive family history. The use of mammography for screening puposes is not recommended before the age of 35 due to the high density of breast tissue in this population.
furthermore, in women with a low or average risk for breast cancer the USPSTF recommends against the use of chemoprevention (tamoxifen, raloxifene) in this group. the USPSTF does, however, recommend that clinicians discuss chemoprevention in those women of high risk group. this recommendation is based on the finding that tamoxifen can significantly reduce the risk of invasive estrogen receptor positive breast cancer. less evidence was found for raloxifene. the USPSTF found good evidence that both tamoxifen and raloxifene increase the risk of thromboembolitic events (stroke, DVT, PE). they found that tamoxifen increases the risk of endometrial CA.
the USPSTF recommends screening for colon CA starting at age 50 in men and women. earlier screening recommendations depend on the patient, family history and medical conditions (ie they are different for conditions such as ulcerative colitis, etc).
the USPSTF recommends against the screening for ovarian cancer.
i wrote these off of the top of my head, so there may be some errors.
to answer your question about your nieces and nephews, the above recommendation is currently what most clinicians will follow. if genetic testing shows a susceptibility to certain cancers, that may change the management. these decisions are usually made on an individual basis.
hope it helped.
What the USPTF won't tell you is that with genetic factors, earlier screening is advisable even if there's not a national preventative guideline for it that early.
For example, if a genome analysis shows increased risk of prostate cancer, UCSF genetics counselors have suggested that screening starting at age 40 may be reasonable.
There are no firm guidelines for early testing, it's a case-by-case basis. The risk of early testing is that you could have a false positive screening test, meaning that the test says you could have cancer whereas you really don't and wouldn't want to have the risk that comes with the tests to make sure you don't have cancer (which can be damaging).