Hi Carly!

This depends on the type of forum (correct  ) you are using. Most of them do offer a way to restrict certain threads or categories depending on either the user's post count, rank, or combination thereof.

If you let us know exactly which script you're using we can probably provide some insight on how to do this.  

EDIT: Just saw your forum  . There's an addon from the official phpBB site that automatically adds members to a user group once they reach a post count. By setting this up, and restricting a forum or category to that group only in your admin you should be able to achieve the result you are looking for! Link below:

http://www.phpbb.com/community/viewtopic.php?t=141342

I hope this helps!