When it comes to marriage, members of the millenial generation are saying "I don't" more often than their elders.

A new Urban Institute report says today's young American adults are on track to have the lowest rates of marriage by age 40 compared to any previous generation.

The report says that if the current pace continues, more than 30 percent of millenial women will remain unmarried by age 40. That's nearly twice the share of their Generation-X counterparts.

Experts point to several reasons behind the plunge in taking the plunge. More American couples are living together without getting married, and some are raising families. Researchers also say that while marriage used to be the starting point for young adults, many people now feel they should be more established, particularly financially, before they walk down the aisle.

The lower marriage rate could have an economic downside: married couples tend to be better off financially, which means they have more spending power.