Yes.  You need to be 13 or in high school.  But just because Facebook says they should be at least 13, you need to decide as the parent if they are ready for the technology.  First, do you know how Facebook works?  If not, spend some time getting to understand what your child will be using it for.  Check out the privacy settings, make sure you know the difference between the wall and private messages and read the Dos and Don’t of Facebook.  Then talk to your child about everything you learned.  Ask them who they will be friending, how much time they will spend using the site, what kinds of things will they be putting on the site about themselves, etc.  If you feel they understand their responsibility, only then are ready for the technology.