The answer to your question depends on the chapter in which a player would be participating. Some chapters are 18+, some you have to be 13 or 14, others you can be a little younger though combat would be limited.
If you go to the main page and scroll down, there's a list of individual boards for all the chapters. Most have age policies posted, sometimes they're in with the general chapter policies and require a little scrolling. If you throw a post up in the local out of game forum, I bet you'll get a detailed answer (especially in chapters where there's a range of paging possibilities for younger ages).