The shadow culture secretary complained that as women age they are "pushed out of the door" as she addressed TV executives.
At the Royal Television Society Cambridge Convention, she said: "Something seems to happen when they reach their 50th birthday - it's like the viewer needs to be protected from the sight of them."
The issue of fewer older female presenters has been levelled at broadcasters a number of times in recent years and the BBC has pledged to change the balance.
BBC1 is broadcasting a series of Rip Off Britain hosted by Julia Somerville, 66, Angela Rippon, 68 and Gloria Hunniford, 73.
Ms Harman pointed out that TV had prided itself on drawing on abilities from people in all walks of life, citing Lord Bragg and Greg Dyke among those who had risen to prominent positions from humble backgrounds.
But she warned "that appears to be decreasingly the case now" and suggested there was a strong element of nepotism for those coming into the industry.
"There has been a faster shift away from social exclusivity than any other profession," Ms Harman said.
"Television, beloved by people from all parts of this country and from all walks of life, must draw on the talent of people from all parts of this country and from all walks of life."
She said: "Whenever I go to television interviews I always take note of the young person who takes me in, often on work experience or doing an internship. So often it's the son or daughter of someone who works there."
She voiced concerns about excessive BBC pay-offs and hefty executive salaries after an appearance by ex-BBC director-general Mark Thompson and BBC Trust members, among others, before the public accounts committee as MPs looked into hefty pay-offs.