According to the Bloomberg Q&A with the Facebook founder:
Facebook isn’t a suitable environment for children under the age of 13, but neither is it appropriate to try to completely shield minors from the rough-and-tumble online world, Mark Zuckerberg told participants at a public question-and-answer session. “I would want my children to use technology because I think it’s one of the ways that you become literate and learn the skills that you need to for the modern world,” he said.
One woman asked Zuckerberg, “If you and I were married, how would we handle Facebook with our daughter?” He responded that it would depend on whether the daughter is 13 yet — that’s Facebook’s required minimum age to become a user.
“I would not allow my child under the age of 13 to use Facebook,” he said. After she was old enough, they would sit down and have a talk about bullying and other things to watch out for on social media, he said.