In 1990, U.S. homelessness was most common among people in their 30s. In 2000, it was most common among people in their 40s. In 2010, it was most common among people in their 50s.

Every decade, the group facing the highest risk of homelessness was born between 1954 and 1963.

Problems facing this cohort of late Baby Boomers were discussed in a 2013 study published in the "Analyses of Social Issues and Public Policy." In short, they came of age in the late 70s and early 80s in a period of depressed wages for unskilled workers, higher youth and young adult unemployment, and rising rental housing costs. At the same time, they faced a proliferation of crack cocaine, leading to social problems and incarceration.

"These conditions could have created an underlying vulnerability that resulted in a sustained risk for housing instability over the ensuing decades," the study concluded.

Here's a chart adapted from the study showing this disturbing trend:

A large population of aging homeless people is a worrying prospect. So is the idea that a similar cycle could be happening again.

Millennials are also facing high unemployment and a steep rental housing market, while heroin and prescription drugs are becoming increasingly prevalent.

Dennis P. Culhane, a professor at the University of Pennsylvania who worked on the homelessness study, told Business Insider via email that millennials in their 20s "are not yet appearing as homeless at rates greater than we would expect based on their representation in the population" but that a similar lost cohort could emerge.

"All of the risk factors do seem to be present for another wave of adult homelessness," Culhane said. "And, indeed, while we don't see rates exceeding expectations yet, we do see that the proportion of homeless in their 20s is growing every year for the last five years, and that they now account for 25% of the adult homeless population."

Homelessness has been declining since 2007, but it remains a significant problem — on a single January night in 2013, 610,000 people were homeless in the U.S.

