If I had my choice, the age limit would be 18yrs. Don't like what it can encourage in young lives and if children were not allowed on facebook, there may be a lot less perverts being able to groom kids.
It can be an evil tool, even if it can also be informative, although generally speaking, the same information can be found on other internet sites.
The children see their school friends all day, surely that is enough. Outside interests would be far more beneficial at such young ages and maybe expand their friends and experiences beyond the school playground.
I hope my grand children are not allowed to use it.
Humankind has not woven the web of life. We are but one thread within it. Whatever we do to the web, we do to ourselves. All things are bound together. All things connect.