Social Media Platforms have minimum age requirements. Do you know what they are?
If you or your child lies about their age and says that they are 13 to enable them to join Facebook when they are only 9, then when they are 14 Facebook will show them as being 18. This could lead to a stranger contacting them, believing them to be over the age of 18, and attempt to form a relationship. This could have significant consequences for both parties.