[Opserty]

Even locking the age is useless, they can just sign up using another email and username. There is no sure fire way of restricting users, display a disclaimer page of some sort before a user enters the forum. (I.E. they must state they are over 18 and accept responsibility for what they are doing) Provide both the option to continue or go back.

You can set-up different usergroups and put under-17s in one and over 18s in the other to limit the under 17s awareness of the over 18s forum existence but someone is bound to find out and tell but I guess it is a preventative measure.