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They are members of the largest generation in history — more than 3 billion people worldwide under the age of 25. About 1.2 billion of them are adolescents just entering their reproductive years.

If they choose, collectively, to have smaller families than their elders did, the world's population — now 7 billion — will continue to grow, but more slowly.

According to United Nations projections, the number will rise to 9.3 billion by 2050 — the equivalent of adding another India and China to the world.