'No religion' third world group after Christians, Muslims
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An age breakdown showed Muslims had the lowest median age at 23 years, compared to 28 for believers all around the world. The median age highlights the population bulge at the point where half the population is above and half below that number.
"Muslims are going to grow as a share of the world's population and an important part of that is this young age structure," Hackett said.
By contrast, Judaism, which has 14 million adherents or 0.2 percent of the world population, has the highest median age at 36, meaning its growth prospects are weakest.
Hackett noted that Israel, which has 40.5 percent of the world Jewish population, had a younger age structure than the United States, where 41.1 percent of the world's Jews live.
Global Christianity's median age is 30 and Hinduism's 26. With a median age of 34, the growth prospects for religiously unaffiliated people are weak, the study showed.
The study estimated Christianity was the largest faith at 2.2 billion adherents or 31.5 percent of the world's population. The Roman Catholic Church makes up 50 percent of that total, with Protestants – including Anglicans and non-denominational churches – at 37 percent and Orthodox at 12 percent.
There are about 1.6 billion Muslims around the world, or 23 percent of the global population. "The overwhelming majority (87-90 percent) are Sunnis, about 10-13 percent are Shia Muslims," the study said.
Among the 1.1 billion unaffiliated people around the world, 62 percent live in China alone and they make up 52.2 percent of the Chinese population.
Japan is the only other country with an unaffiliated majority, at 57 percent of the national population. After that comes the United States, where 16.4 percent of all Americans said they have no link to an established faith.
The world's Hindu population is concentrated mostly in India, Nepal and Bangladesh. Half of the world's Buddhists live in China, followed far behind by Thailand at 13.2 percent of the world Buddhist population and Japan with 9.4 percent.