WASHINGTON — The work ethic is such a central part of the American character that it's hard to imagine it fading. But that's what seems to be happening in one important part of the labor force. Among men 25 to 54 years old — so-called prime-age male workers — about 1 in 8 are dropouts. They don't have a job and, unlike the officially unemployed, aren't looking for one. They number about 7 million.