In this file photo, President Barack Obama gestures as he speaks at the University at Buffalo, the State University of New York, in August 2013, when he began his two-day bus tour to speak about college financial aid. (AP Photo/Keith Srakocic)

For much of the 1990s and the first decade of this century, college enrollment in the United States surged as the number of high school graduates ticked upward thanks to the Baby Boom Echo.

The rise in the number of students on college campuses was propelled in part by affluent, well-prepared high school graduates of all races. Colleges not only filled their seats with students who could afford to pay a large part of the tuition bill, but institutions also redirected some of that money to students with financial need.

Those days are over in more ways than one.

First, the number of high-school graduates in the U.S. has been declining since 2011, according to the Western Interstate Commission on Higher Education, which tracks national trends. Even when the number of graduates begins to rise again in 2018, it will remain below the high of 2011.

Colleges enroll more than just recent high-school graduates, of course, and the market for older non-traditional students is growing. But most colleges and universities unfortunately have designed their academic programs primarily to attract 18 year olds, and as a result, the competition for traditional college students is turning into a fierce recruiting game.

Not only is the overall pool of high-school graduates shrinking, but so too is the number of college-going students able to pay ever-increasing tuition prices.

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And it’s not going to get any better in the coming decade. Of the 450 counties in the United States with significantly more younger children than older ones right now, all but 100 of them have median incomes below the national average, according to an analysis by the Chronicle of Higher Education.

Such demographic trends have the potential to widen the growing divide between the rich and poor when it comes to who goes to college. Not every high-school graduate needs to attend what Americans typically define as college — a four-year residential campus — but today’s ever-evolving economy demands some sort of education after high school.

Right now, whether high-school graduates go to college depends almost entirely on just one factor: how much money their parents make. Seven out of every 10 students who come from high-income families and score in the top quartile of standardized tests earn a bachelors degree, compared to just 4 of 10 students from low-income families with the same academic background, according to a new working paper published this month by the National Bureau of Economic Research.

That paper — by Lindsay Page of the University of Pittsburgh and Judith Scott-Clayton of Teachers College, Columbia University — found that the real problem isn’t the lack of aid for needy students, but rather the complex financial-aid system that often confuses even parents who have navigated it for multiple children.

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The proliferation of federal and state programs, plus those on individual campuses, all with their own application deadlines and requirements, often leads low-income students to make decisions that are not in their best economic or academic interest because they don’t have a network of family and school advisers to help them.

For example, a surprisingly large share of low-income students who place a deposit for college in the spring of their senior year of high school fail to show up on campus the following fall because they missed a financial-aid deadline. The authors noted that the summer between high school and college is unique because students no longer have ties to their high-school guidance counselors and they don’t yet have access to their college advisers.

“A dominant theme that emerges is the overall complexity of the college transition,” the authors wrote. “Solutions that focus on just one type of barrier — such as college affordability — may lead to improved access, but may not be the most effective use of resources if other challenges still stand in students’ way.”

Until recently, given the glut of students, it was easier for many colleges to focus their limited financial resources on serving students able to easily navigate the complex financial aid maze that brought them to campus.

But with the demographic trends indicating that more and more families will have difficulty affording college in the coming decade, this approach no longer works. There are simply not enough well-prepared, well-off students anymore to fill all the seats available at American colleges and universities.