Does anyone know how you find out the year a building was made?

You could try to get information from your local cadaster.

Look for the little stone above the door that says erected in the year of lord 1348 (or whatever).

Try calling the Department of Buildings on Monday. Here is the number for the Brooklyn office. (718) 802–3675

Put the address in the query on the right side of the linked page and see it it helps.

http://www.nyc.gov/html/dob/html/home/home.shtml

just pick it up and look – it’s usually on the bottom

The style of the building usually gives it away.