21 South End Avenue was built in 1988 — roughly 38 years old. For property-tax purposes it falls under Class 2 (residential buildings with four or more units, condos and co-ops), taxed at 12.439% of a taxable value based on 45% of the city’s income-derived market value.
Data: NYC Department of Finance and Dept. of City Planning (PLUTO) via NYC Open Data. Figures are public records, may lag current bills, and can exclude abatements. Not financial advice.