U.S. student loans have topped the $1 trillion mark, according to the Federal Reserve Bank of New York, with two-thirds of the debt carried by borrowers under age 40. The mounting burden is putting the brakes on the housing rebound, as hefty payments each month or delinquencies on student loans disqualify many young people from taking advantage of today's rock-bottom mortgage rates and bargain home prices. Buying is more affordable now than renting in many housing markets, yet student loan borrowers are being held back "because all their money is going to pay off their student loans," says University of Illinois law professor Robert Lawless. As a result, the share of under-40 homeowners shrank by 4.6 percent between the third and fourth quarters for the largest decline since recordkeeping began in 1982. When first-time buyers do not enter the market, move-up buyers also are put on hold -- all of which stunts growth in the housing market. Not only is the residential property market suffering, adds Diane Swonk to Mesirow Financial, so is the broader national economy. "Student debt has a dramatic impact on the ability to buy a house, and to buy the dishwashers and the lawn mowers and all the other purchases that stem from that," says the economist. "It has a ripple effect throughout the economy."