In Ben McConnell and Jackie Huba's Citizen Marketers they state that "content creators online tend to be young, and they're getting younger."
Today social media and the world wide web are so connected it is normal and almost natural for an internet user to eventually add something to the webscape. Nearly everything is accessible when you sit down at a computer, add in cellphones (ownership of cellphones among 4-14 year-olds has shot up 50% since 2005), and you can have middle schoolers updating their Twitter and Xanga accounts over lunch.
One new kid on the web, that of whom I'm a fan, is Tavi. Tavi Gevinson started her blog "Style Rookie" in March of 2008, when she was 11. She also works for British magazine Pop where she writes another mostly photo blog, and has been featured on the cover. She's a superstar in Japan, and a "dork" as she claims on her blog. Check her out at fashion week here! Also... she's the inspiration for the new Rodarte line at Target.
Pretty cool for 13. I think.