The middle class is on the decline across the wealthiest countries in the world and the shift is hitting the youngest generations hardest, according to a new report published by the Organization for Economic Cooperation and Development (OECD). Today, just 60% of millennials (those born between 1983 and 2002) are considered middle-class, compared to 70% of baby boomers (those born between 1943 and 1964) when they were in their twenties, according to the report, "Under Pressure: The Squeezed Middle Class." Worldwide, the share of households considered middle-class has slipped from 64% in the 1980s to 61% today. The report, which analyzed the 36 largest economies in the world, defines the middle class as those earning between 75% to 200% of the median national income. In the U.S., that translates to annual earnings of between $23,416 and $62,442 for a single worker.

Since the baby boomer generation, the middle-income group has grown smaller with each successive generation. 'Under Pressure: The Squeezed Middle Class'

Income inequality is a driving factor, the report says. It explains that while average earnings have stagnated or barely risen in developed nations over the past 30 years, the cost of living has increased dramatically, while jobs have become less stable. At the same time, the richest households are accumulating even more wealth. "Current findings reveal that the top 10% of in the income distribution holds almost half of the total wealth, while the bottom 40% accounts for only 3%," the OECD reports. As the rich get richer, in other words, the middle class is hollowing out.

Millennials are worse off than their parents were

Although the middle class makes up more than half of the population of OECD countries, it is shrinking. In fact, "since the baby boomer generation, the middle-income group has grown smaller with each successive generation," the report notes. Because of that, young people are carrying increasing amounts of debt and being priced out of the housing market. That's due, in part, to income stagnation. Median wages grew by an average of 0.3% per year between 2007 and 2017, according to the report, compared to more than three times that between the mid-1980s and mid-1990s. Meanwhile, the cost of living continues to increase. Housing costs, in particular, account for a larger share of a middle-class family's budget. Home prices have grown "twice as fast as inflation and one-and-a-half times faster than the household median income," per the report.

Housing also accounts for more than one-third of a family's spending, compared to just a quarter in 1995. Health care and education costs have also outpaced inflation, particularly in the U.S. This has left many in the middle class struggling to pay their bills day-to day: More than 20% of middle-income households spend more than they earn, the report says.

Young people are hit the hardest