The British star was 23 when she filmed her scenes as 16-year-old Jenny, who is romanced by an older man, played by Peter Sarsgaard, and she won an Oscar nomination for her performance.
Schwimmer, whose new film Trust shows a teenager lured to a motel for sex by an online predator old enough to be her father, admits he was unsettled by Mulligan's casting in An Education, and has taken aim at film chiefs for not hiring a younger actress.
He tells London's Time Out magazine, "I think she's (Mulligan) a brilliant actress, but I think it's irresponsible casting. If you're telling a story about a minor, a 15-or-16-year-old girl who has a relationship with a 40-year-old man, I think to cast a young woman who is considerably older than that is... dangerous.
"The audience, unconsciously, becomes completely accepting. It's not weird or strange or uncomfortable or immoral. There's no grey area. Carey was in her early twenties when making that film, and even if she's playing young or made to look young, she's not 16.
"In my mind, there's a huge difference between a 16-year-old actress and a 20-to-21-year-old actress, and if your story hinges on that age difference, then cast a young actress.
"I guess I just feel it doesn't hold up in court. In my court, anyway. Even if a 15-year-old is mature beyond her years, then cast a 15-year-old girl who's mature beyond her years. I just think you need to be more responsible."