The 1946 to '64 postwar craze for making babies created a generation that has been famously changing America for 60 years. But one part of the baby boom, the part born in the early 1960s, remains quietly obscure. These boomers came along too late to join civil rights marches, fight or protest the Vietnam War, exercise their flower power, build geodesic domes out of peach crates, or even feel terrible when the Beatles broke up. There are 21 million of them. They make up more than a quarter of the 76 million-strong baby boom. The youngest will be 50 this December.