U.S. recessions since the oil crisis in the early 1970s each had their own special causes and victims, but they also had something in common: They were over relatively quickly. The current downturn, however, is deeper and already longer than any since World War II. This spells trouble for one especially vulnerable group — managers in their 40s and early 50s. They tend to be more expensive than their younger counterparts; they may lack some of the high-tech savvy needed to succeed in a more efficient workplace; and they face a downsized job market that will stay that way much longer than usual.  Even in a “normal” downturn, the job market is a “lagging indicator” (meaning it does not show improvement for several quarters after the start of a recovery). And the current recession is anything but normal. According to Wharton finance and statistics professor Francis X.…