Caitlin Juetten couldn't find a job for a year after she graduated from New York University with a political science degree. She eventually landed a social media job for a software company, was later laid off, and is a nanny now. She may go to graduate school and move in with her parents. Her boyfriend, a writer, is also unemployed and lives with his parents.

"I'm not going to get married for a while," Juetten says. "I used to think that by the time I'm 30, I'd have kids. Now that I'm 25, I'm thinking this is never happening ... I'm not even going to get married until I get health insurance."

The United States' 86 million millennials — more numerous than baby boomers — may alter the growth trajectory of the U.S. population for years to come.

The United States has been the envy of the developed world for more than 20 years in the way its fertility rate has stayed near the 2.1 percent replacement level — the rate needed to keep the population stable.

But the average number of births per woman has fallen 12 percent from a peak of 2.12 in 2007 to its lowest in 25 years.

Deaths among whites who are not Hispanic outnumbered births for the first time ever last year, a sign that the country may face an age imbalance that could threaten its ability to support a growing aging population. The oldest of 77 million boomers turn 67 this year. The oldest millennials are 31.

The recession has taken a toll and created more obstacles for a generation already cool to the idea of matrimony. Mired in student-loan debt, struggling to find work and often forced to live with their parents, millennials have plenty of reasons to postpone having kids.

Adding to the decline: Fewer immigrants are coming into the country because of poor job prospects. Members of the largest group — Hispanics — have helped prop up the national birth rate because they are younger and have more children. But last year, Hispanic births declined at an even faster rate than non-Hispanic whites.

"The recession had a big impact — a double-barreled impact," said William Frey, demographer at the Brookings Institution in Washington, D.C. "It affected people here in the United States, delaying marriage along with childbearing, buying a home. And immigration slowed down."

A fertility rate at replacement level is a sign of economic vitality for developed nations. Without that balance, countries are left with an older population and not enough young people who can work and support social programs to care for the elderly.

It's a huge challenge for countries such as Italy and Japan, where rates are as low as 1.4. In Japan, deaths outnumbered births nationwide last year. Some countries, including Germany and Russia, among other countries, offer financial incentives to encourage women to have babies.