They last just about forever, and aren't ever torn down because they're old. The Woolworth Building is undergoing its zillionth restoration, and it was built in maybe around 1890. The first really tall steel-frame (actually it's wrought iron, but okay) structure was the Eiffel Tower, and it's doing just fine. As for stone construction, the old cathedrals in Europe have been there for many hundreds of years.
Normally skyscrapers are torn down because the land they're sitting on is more valuable for something else, usually a taller building. The World Trade Center situation was, of course, a unique situation; the Deutsche Bank building was probably restorable, but it was more feasible economically just to take it down and start over. Except for that one, all the buildings which weren't destroyed outright were found to be perfectly sound from a structural standpoint and were simply restored with new curtain walls, windows, and interiors.