Older buildings, at least those built prior to WWII, are specifically sought out because they don't have paper thin walls and floors. They generally have plaster interior walls, thick poured concrete floors, and triple layer masonry exterior walls. Postwar buildings on the other hand tend to have gypsum (sheetrock) interior walls, thinner reinforced concrete floors acoustically bound to the frame, and thinner exterior walls. Newer windows are much quieter, but that's something that can be updated fairly easily.

The worst buildings were probably built from the end of WWII to the early 1970s. Stuyvesant Town apartments, for example, are terribly noisy.