Median age

The age at which half of a place's residents are younger and half are older.

The Census Bureau asks every person in a household for their date of birth and derives age in completed years as of the survey reference date: age is "the person's age as of the last birthday", and a person's reported age and date of birth are reconciled by the Bureau before publication (ACS Subject Definitions). The published median is the single value that divides the resident population into two halves of equal size, computed by the Bureau from the grouped age distribution rather than from individual records.

The denominator is everyone counted as a resident under the Bureau's residence rules, not only adults and not only householders. Students in dormitories, residents of nursing homes and other people in group quarters are residents of the place they are counted in, so a place built around a campus or a retirement community carries their ages in its median.

A median is a position, not an average, which is why it is the measure published here. One very old or very young resident moves an average and leaves a median where it was, so the median describes the typical resident of a small place far more stably than a mean would.