Even as the average U.S. household pares down its debts, the new degree-holders who represent the country’s best hope for future prosperity are headed in the opposite direction. With tuition rising at an annual rate of about 5% and cash-strapped parents less able to help, the mean student-debt burden at graduation will reach nearly $18,000 this year, estimates Mark Kantrowitz, publisher of student-aid websites Fastweb.com and FinAid.org. Together with loans parents take on to finance their children’s college educations — loans that the students often pay themselves – the estimate comes to about $22,900. That’s 8% more than last year and, in inflation-adjusted terms, 47% more than a decade ago. ...