How young can a child be to use Facebook?
On the Facebook website, they write "Facebook requires everyone to be at least 13 years old before they can create an account (in some jurisdictions, this age limit may be higher). Creating an account with false info is a violation of our terms. This includes accounts registered on the behalf of someone under 13."
However, there are children younger than 13 who do use Facebook. Facebook does not verify that the year of birth that someone enters when creating an account is correct. Therefore, you need to be aware of what your child is doing online since they can easily create an account regardless of their age.
But once they are 13 or older, when should they have an account? The time for an account is when you feel that they are responsible enough to manage the social, privacy, safety and security issues that they'll face on Facebook, or other social media applications - with and without your monitoring.