(From HBR Blog Network) -- The population used to be shaped like a pyramid: lots of young people, a medium number of middle aged, and a few old folks. But the demographic geometry has changed radically in just the last few decades in many parts of the world — and will shift further over the decades ahead in still others. We now have diamond- or rectangular-shaped populations in many countries and will at some point have inverted pyramids — the old will outnumber the young.

The United Nations' most recent study on demographic trends confirms these changes and puts to rest any assumption that the pyramid-shape will return. The former ratio of old-to-young already no longer exists in many countries and, much of the world will soon follow. Yet many of our talent management practices today are derived from this old idea.

A combination of lower birth rates and longer life expectancies has conspired to create new geometric shapes. These two major demographic shifts are so significant that Peter Drucker predicted that historians, looking back at the 20th Century, will view the demographic changes as the most important events of the century (more so than technology, industrialization, globalization and so on).

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