The Utah Supreme Court recently adopted a bright-line cutoff for liability for negligent children, setting the limit at 5 years old. The case involved a 4 year old who threw a rubber dolphin at his babysitter. Unfortunately, the dolphin hit the babysitter in the eye. She had just had a cornea transplant and the accident blinded her. The Volokh Conspiracy has the story.
Setting an age limit is not that unusual, but the age itself varies among states. I have read somewhere that the more common age is 7, but I have not done the research to confirm it. The Restatement (Third) of Torts suggests the age of 5, which is one of the reasons the Utah Supreme Court chose it.
One interesting thing about age cut offs is that the most famous case regarding intent (as an element for intentional torts) involved a 5 year old. Remember Garratt v. Dailey? I am sure you read it in law school. It's the case case about the 5 year old who pulls a chair out from under a woman who was about to sit down on it.
Thanks to TortsProf for the information.