(Disclaimer - I am not an investigative journalist. Everything here is part speculation, part nagging suspicion, and part whatever I was able to cull together in one afternoon. No actual science was involved.)
As a woman, and one who is closer to her forties than her twenties, I have more than a passing interest in how sexism and ageism intersect in the music blogosphere. This link a grammar, a writer for Pitchfork, left on his tumblr says that the popular music blog has a bias against music made by women over the age of forty. According to Flavorwire's research, Pitchfork's reviews for a handful of recent albums by female artists are substantially lower than the averages given by Metacritic which pulls in reviews from many sources. Some of the examples given are Hole's Nobody's Daughter, which scored a 55/100 according to Metacritic, and a 29/100 from Pitchfork. Madonna's Hard Candy gets a Metacritic score of 65%, almost 10% higher than Pitchfork's.
Yeah, that smells a little like bias. But wait. a grammar says their research is "is missing several control groups, including but not limited to: (a) how Pitchfork ratings as a whole compare to Metacritic averages, and (b) how Pitchfork ratings of men over 40 compare to Metacritic averages, plus also things like (c) genres/label-origins/how-long-they’ve-been-making-music of Pitchfork-reviewed albums by men/women over 40, and how they compare, etc. (Is there a male Madonna to compare with here? Is it George Michael? A male Liz Phair?) See also: how do Pitchfork ratings for albums with Metacritic scores compare to Pitchfork ratings for albums not indexed on Metacritic? Let’s not invoke science unless we’re invoking SCIENCE!"
Because I am a nerd like that, I tallied some of the scores for albums made by men over forty, and compared them to Metacritic's cumulative score:
Wilco's The Album
Pitchfork - 73%
Metacritic - 76%
Morrissey's Year of Refusal
Pitchfork - 81&
Metacritic - 79%
Bruce Sprinsteen's Working on a Dream
Pitchfork - 58%
Metacritic - 72%
Pearl Jam's Backspace
Pitchfork - 46%
Metacritic - 79%
To be honest, I had to do a bit of digging to find Pitchfork reviews of albums made by artists over forty of either gender. That itself should be addressed, but from the few I did find, I'll hazard a guess that a) Pitchfork does have some bias against music made by women over forty, b) Pitchfork has some bias against music made by middle-aged people regardless of gender, or most likely, c) Pitchfork has some bias against artists not currently in the indie rock, pop, or hip-hop pantheon. (Wilco and Morrissey remain in the critics' good graces despite crossing over into "elder statesmen" territory, and among the albums made by women, Laurie Anderson's reviews were consistent across the board.)
This is not to pick on Pitchfork -- I think you would probably get the same results from any popular music blog. We live in an ageist, sexist society, and our popular culture unfortunately reflects that.