By Patricia Reaney

NEW YORK (Reuters) - More Americans 50 years and older are copying younger generations and eschewing marriage, opting instead to live with their partners, according to new research.

In 2016 about 18 million Americans were cohabiting, defined as living with an unmarried partner, and nearly a quarter of them were people over 50, an increase of 75 percent since 2007, data released on Thursday from Pew Research Centre showed.

"Baby Boomers have a higher divorce rate and there are a greater number of unmarried people in that age group" than previously, Pew research analyst Renee Stepler said in an interview Thursday.

Government figures show that so-called “grey divorce,” or splits among adults 50 and over, has about doubled since the 1990s and could partly account for the increase in cohabitation.

Fewer marriages, changing social norms and women's greater economic independence are other explanations for the rise, Stepler added.

As cohabiting has gone up, the marriage rate in the United States has dropped, from 8.2 per 1,000 population in 2000 to 6.9 in 2014, according to figures from the Centres for Disease Control and Prevention.

Stepler also pointed to an increase in the number of older Americans who have never married. Pew found that 27 percent of people 50 years and older who are cohabiting have never married, while more than half are divorced and 13 percent are widowed.

In younger age groups, the majority of cohabiting adults have never tied the knot: 97 percent of 18-24 year olds and 85 percent of 25-34 year olds.

Although cohabitation rates are rising, cohabiting couples account for only about 7 percent of the overall U.S. population and 4 percent of over-50s.

Most older cohabiting couples were in their 50s. But nearly 30 percent of them were in their 60s, 10 percent in their 70s and 3 percent were 80 years or older.

Pew Research Centre compiled its findings on cohabiting by analysing data from U.S. Census Bureau and the Current Population Survey, which included information on 134,562 adults ages 18 and older. The survey is sponsored jointly by the Census Bureau and the U.S. Bureau of Labour Statistics.

(Reporting by Patricia Reaney; Editing by Patrick Enright and Andrew Hay)