Some quick guidance from "Action for Children".

Please bear in mind, if you sign up for these sites, and don't tell the truth about your age, that information may follow you on social media. For example, if you sign up for Facebook aged 8, stating you are 13, by the time you are actually 13, both Facebook and the outside world will believe you are 18. Would parents be happy with their 13 year old children receiving content and messages aimed at 18 year olds?