One grim general conclusion was shared by two studies of the U.S. economy this Labor Day: The American Dream is fading for many people, especially young couples, as the rich get richer and the poor get poorer.

Though President Reagan's administration has accented the nation's 68-month economic expansion, the Economic Policy Institute and the Children's Defense Fund both noted common administration data in their separate conclusions.The research and advocacy organizations found the situation for young couples particularly tough, with the EPI saying that the average family in 1987 led by someone between age 25 and 34 had an income 9 percent lower than counterparts in 1973 - and the CDF reporting that the poverty rate for young families rose from 12 percent in 1973 to 22 percent in 1986.

Three-fourths of that poverty increase came during the 1980s, according to the CDF report prepared jointly with the Center for Labor Market Studies at Northeastern University in Boston.

"The fact that a much smaller proportion of young families have been able to buy homes (recently) is symbolic of their diminished economic well-being and opportunities," said the EPI study.

"This new poverty reflects the expansion of low-wage jobs, cutbacks in government cash assistance to the poor, the collapse of the purchasing power of the minimum wage and continued race and sex discrimination," it charged.

Despite the liberal leanings of both organizations, the non-partisan numbers they used could provide ammunition for the Democratic presidential campaign of Massachusetts Gov. Michael Dukakis, who has appealed to those hit by what some analysts call "a rolling depression" in the U.S. economy.