Back in the 1960s and 1970s, when juvenile delinquents were called juvenile delinquents, youth gangs stuck close to home and defended their turf against invasion by other gangs. No more. The age of the mobile youth gang has arrived. Striking out from poorer neighborhoods, they are fanning into wealthy areas and often faraway towns, breaking into jewelry stores and snatching wallets from pedestrians.
While present to some extent in a number of big U.S. cities, the mobile predators are most evident in the Los Angeles area, where police have identified no fewer than 450 gangs with more than 45,000 members. This...