A white picket fence, a well-manicured lawn and a cute little house owned by ... a recently graduated twentysomething? Not so much. The suburbs, which since World War II have dominated the American real estate scene, are suffering an identity crisis. Baby boomers are nearing retirement age, people are marrying and having kids later in life, and college graduates are flocking to cities. Housing experts say these cultural and aging shifts have slammed the door on the era of urban sprawl.