I was actually a strong supporter when Facebook stated they wanted to create a children's only social media platform. I know it would not have been perfect, but I also believe kids need some exposure to the tech world that they will be thrown into as a teenager. Sadly the backlash was too much and as far as I know it is still a dead project.
I think 16 years would be the most appropriate age for using social media. In our country, you can become a citizen of the country and get citizenship documents after you are 16.