Go to your:
Admin CP -> Users -> Node Permissions -> [click the forum]
Make it a private node and click
Update:
View attachment 11559
Then grant access to the
Registered group (in the same area):
Admin CP -> Users -> Node Permissions -> [click the forum] -> [click Registered]
Set
Allow permissions.
The nature of a private node is that everyone is denied access except those which you explicitly allow.
If you can think of a way to verify people's age during registration...
You might consider just adding a notice with a warning that you must be 18 years or older. See this thread:
http://xenforo.com/community/threads/message-for-your-guests.5934/