Yards may be a little overgrown and groceries may take a little longer to find their way into bags this summer as many teens search for seasonal employment. According to a study by Northeastern University’s Center for Labor Market Studies, about 5.1 million, or just 29.6 percent, of 16-to-19 year olds were employed last summer nationally, the lowest since World War II. That number was at its peak in 1978 with nearly 60 percent of teenagers having summer employment. From then, the number hovered around 50 percent until 2001.