All discussion forums, including VB/Xenforo like this one, have the option to include the age limit of 13 when the template is being built...the admin/builder can choose what he/she wants from the setup and membership "rulebook" tick boxes. Doesn't matter where in the world the site is based, the template is the same and the choices for setup are the same. This one "should" have the age limit of 13 box ticked - when registering a potential new user will be asked if they are over 13 if the box has been ticked on initial build setup by the admin
Of course there are those who will make up a date of birth in order to bypass the 13 years old, nothing anyone can do about that
Also on all forums there is another tick box for the builder to vet all membership applications before confirming...I specifically liked that option as it made IP tracing and behaviour vetting easier so I could refuse applications to join on the grounds of being a known spammer or troublemaker. There are several check sites that an admin can use to cross reference IP's and usernames to see if anyone joining (or already joined who is causing concern) has a track record of being banned or getting warnings for bad language/behaviour.
No posts on a forum are ever deleted, they remain on the site memory for as long as the site is in existance and even after a site is defunct, you can still pull up old posts from years after a site is taken down.
(I spent a long time building forums for an owner based in the US, but although I am UK based, the software is universal, so that 13 and over rule is there to be used when building all forums)