Population measures provide insight on the changing size and face of communities, and an area's needs and assets. For example, areas with large or growing populations of seniors or children may face challenges in providing needed services and generating enough tax revenue to support those services.

The population of Westchester County has been aging. Between 2000 and 2016-20, the number of people aged 85 or older increased 58% and the number of people between the ages of 60 and 84 increased 34%. By comparison, the population 40 to 59 grew by only 6%, and there were decreases of 7% and 5% among people 20 to 39 and among those under 20 respectively.

In general, Westchester County's population has been aging more slowly than similar counties. The growth rate among people aged 85 or older in Westchester was lower compared to Nassau (83%), Rockland (75%) and Putnam (67%) counties, and its growth rate among people ages 60-84 was lower than Putnam (95%) and Rockland (47%), but higher than Nassau (25%).

Among localities in Westchester County, Mount Vernon has had dramatic age shifts with increases among populations aged 85 and older (77%) and 60 to 84 (35%) and a decrease of 25% among those under 20. Most localities lost populations in the 20-39 age range, though Elmsford, Cortlandt, Harrison, New Rochelle, Lewisboro and White Plains had small gains. Harrison had the largest increase among those under 20 (33%).

Individuals age into and out of these groups, and this indicator should not be interpreted as a measure of groups moving to or away from the region. The multiyear figures are from the Census Bureau's American Community Survey. The bureau combined five years of responses to the survey to provide estimates for smaller geographic areas and increase the precision of its estimates. However, because the information came from a survey, the samples responding to the survey were not always large enough to produce reliable results, especially in small geographic areas. CGR has noted on data tables the estimates with relatively large margins of error. Estimates with three asterisks have the largest margins, plus or minus 50% or more of the estimate. Two asterisks mean plus or minus 35%-50%, and one asterisk means plus or minus 20%-35%. For all estimates, the confidence level is 90%, meaning there is 90% probability the true value (if the whole population were surveyed) would be within the margin of error (or confidence interval). The survey provides data on characteristics of the population that used to be collected only during the decennial census. Data for this indicator are released annually in December.