Population screening programmes invite people within a defined age range and not outside it. The boundaries follow from how test accuracy interacts with how common a disease is.
Test accuracy is not fixed in practice
A test's sensitivity and specificity describe how it performs on people who do and do not have a condition, and those properties belong to the test itself.
What a person actually wants to know is different: given a positive result, how likely is it that disease is present.
That answer depends not only on the test but on how common the condition is in the group being tested, which is where age enters.
Rarity generates false alarms
Screen a group in which a disease is rare and most positive results will be false, because the large disease-free majority generates more errors than the small affected minority generates true detections.
The same test applied to an older group, where the condition is far more common, produces a much higher proportion of true positives from identical accuracy.
This is why a lower age boundary exists. Below it, the programme would generate mostly false alarms, each carrying investigation, anxiety and risk.
The upper boundary has a different logic
Disease becomes more common with age, so the statistical case for screening strengthens rather than weakens at the top of the range.
What changes is the time available for benefit. Screening detects disease earlier than symptoms would, and the benefit depends on living long enough for that lead time to matter.
Where competing causes of death are likely to intervene first, detection produces treatment without extending life, which is why upper limits exist.
Overdiagnosis is the hidden cost
Some conditions detected by screening would never have caused symptoms during a person's life, having grown too slowly to matter.
These cases cannot be distinguished from consequential ones at detection, so they are treated, and the treatment carries risk without offsetting benefit.
Overdiagnosis rises with age as slow-growing disease accumulates and remaining lifespan shortens, which pushes in the same direction as the upper cut-off.
Boundaries differ between countries
Different health systems set different ages for the same programme, reflecting differing judgements about acceptable harm, resources and local disease patterns.
Individual risk also varies, and people with strong family history or known genetic factors are often screened outside the standard range through a separate pathway.
Whether an individual should be screened, and when, is a discussion with a doctor rather than something the population boundaries settle.