Among those ages 16 to 19, access to part-time jobs, full-time jobs and summer jobs had already declined through this decade, and by last month only 31.4 percent had held some sort of employment. In the year ahead, Professor Sum predicted, “The teens will be thrown out of the labor market at record levels,” often causing hardship for poor families and causing youths to miss experience that, studies show, will gain them better jobs in the future.