While many cities are quite old in New York, so are many rural areas. Gray areas have no data in YR_BLT field in state tax records. You can't always trust the data though as some town assessor offices will just blank out the form with 1800 or 1850 for old buildings, as the exact year doesn't effect assessed value much.

Here is the Postgres query I used which took a few seconds over the 5.4 million records: SELECT swis, PERCENTILE_CONT(0.5) WITHIN GROUP(ORDER BY yr_blt) FROM nytax_ctr_point GROUP BY swis

Data Source: NYS Statewide Parcel Map Program, gis.ny.gov