age heaping
pp. In a survey result, the clumping of respondents' ages on certain values, particularly those ending in 0 and 5.
According to 2000 census information, there are more than 51,000 Americans who, like Magner, are 100 or older, among them about 1,400 so-called super-centenarians who top 110.

The Census Bureau says the numbers are probably inaccurate, mixing human error with equally human wishful thinking. …

Deliberate age misreporting accounts for the remaining discrepancy between reported and actual numbers of centenarians, experts on aging say.

People may have adjusted their age upward decades ago to enter the workforce, to join the military, to get married or to start receiving benefits, only to have the lie endure.

Researchers also have found that some in their 90s succumb to a phenomenon called "age heaping": rounding up their ages to end in 5 or 0.
—Robin Fields, “Census: 51,000 claim age over 100,” Chicago Tribune, August 30, 2001
1994 (earliest)
The prevalence of "age heaping," or the tendency to over-report ages ending in 0 or 5 is measured here using the Myers blended index and the Whipple index.
—“Demographic Characteristics of Households,” Contemporary Women's Issues, January 01, 1994
Age heaping (which is also just called heaping) occurs because many people (particularly older people) tend not to give their exact age in a survey. Instead, they round their age up or down to the nearest number that ends in 0 or 5. When the ages are graphed, the distribution isn't smooth; instead, there are heaps over the ages ending in 0 and 5.