Demographers see a tipping point in millennials' tendency to avoid marriage

“It’s definitely true that most young adults do still want to marry eventually, and most will, but our data suggests that the landscape has shifted significantly,” said Ms. Parker. Millennials are more accepting of cohabitation and more open to the idea of children being raised outside of marriage.

Still, Pew’s research suggests that only about 25% of those between age 25 to 34 will remain unmarried. That’s higher than in previous generations but suggests the vast majority of millennials will eventually tie the knot.

At any rate, a major shift is coming in how demographers, economists and marketers think about this generation as parents. Since the worst of the recession in 2009, more than 20% of parents under age 34 have been in poverty—higher than any other point in the past 25 years, according to research from Konrad Mugglestone, a policy analyst at Young Invincibles, a Washington-based nonprofit group that researches young Americans. By contrast, only 16% of young people without children were in poverty.