Tomorrow, Barack Obama is expected to announce that (Boston-born) Peter R. Orszog will be his director of the Office of Management and Budget. This is a cabinet-level position, and, if I'm not mistaken, makes Orszog the first post-Baby Boom Cabinet member.
Interestingly, the current OMB director, Jim Nussle, was the first Cabinet director born in the 1960s, according to my rudimentary research. Nussle was born in June 1960. He's not quite the youngest ever, though -- Steven Preston, born August 1960, became HUD Secretary earlier this year.
Tim Geithner, the first actual, official, Cabinet post announcement, is already the youngest ever, at a spritely 47, born in 1961. Still well within Baby Boomer parameters.
But Orszog is by no means Boomer-aged. He was born in December 1968.