Affleck, 41, made the comment on Monday's edition of "Late Night with Jimmy Fallon."
His casting in an as-yet-untitled Batman-Superman movie was met with mixed reactions when it was announced last month, but some of the naysayers have been very vocal about Affleck not being right for the role.
"They called me up and said, 'Do you wanna do this?' And I thought, 'Well, I'm not 25, man. Are you sure about this?'" Affleck told Fallon of his casting as the Caped Crusader in director Zack Snyder's "Man of Steel" follow-up.
The actor went on to say executives at Warner Bros. wanted to be sure he could handle the criticism he would likely get from the media and fans about taking over the iconic role last played by Christian Bale.
"'Listen, we want to talk to you because people go through this process and it can be trying... We want to show you some of the reactions that past people who have been cast have gotten, like on the Internet and stuff,'" he said, quoting the studio bosses. "[They showed me] people who were in these movies who did a great job and [Web commenters would] say, 'Kill him!' You can't say that before a movie comes out. It doesn't matter what you think then, it matters what you think when you see the movie."
Affleck said he unwisely ignored advice to stay off the Internet for a few days after the news of his casting broke to let some of the furor die down.
"I said: 'I'm a big boy. I can handle any snub. I can handle anything,'" Affleck recalled. "I said: 'I handle [expletive.] I'm very tough.' So, I saw the announcement, I look down the thing and I look at the first comment. The first comment goes, 'Nooooooooooooo.' I'm like, 'We're going to be Luddites [people who eschew all modern technology] for a while, kids.'"
The Oscar-winning filmmaker also said he can't wait to see how his three children react when they see him in the movie in two years.
"Every kid should see their dad as a superhero, right? It's so awesome, man," he said. "I'm so excited."