Jeanna Smialek, who covers the U.S. economy for The Times, will be 33 in a few weeks; she is part of a cohort born in 1990 and 1991 that makes up the peak of America’s population. At every life stage, that microgeneration has stretched a system that was often too small to accommodate it, leaving its members — so-called peak millennials — with outsize economic power but also a fight to get ahead.…