Seriously? Legal, in which jurisdiction? You know this web site serves more than just Alabama.
The US has some laws dealing with protecting children, hence the demand that people under 13 not join. And the US is far from the only country with such laws.
Not to mention subject matter. There are things I would be comfortable talking about with other adults, but not with a minor present. I would not want to be held responsible for some kid getting the wrong idea about a controversial topic.
That's why some covens welcome all ages, above 18, or have other restrictions for membership.
Sites do have the ability to set an age limit to join or even to view. Such restrictions are placed to protect children and the help out the site. The age limit has nothing to do with the restraint to knowledge. If you are under 13, you still may view the site and everything on it.
Covens are allowed to have age limits in both real life and on the site. These reasons are for safety and protection in real life. The reasons for covens having them on the site is personal and should be discussed with the leaders of the coven.
No. Private articles have nothing to do with the age limit. They are private and cannot be seen by anyone not in that coven. Articles and spells are kept private for a reason and that reason is something only the coven would know. It belongs to the coven and coven material are usually kept private as people don't want their work to be seen by people they don't know and would like to assist their coven improve. If you would like to see such, you can apply for the coven or talk to its leaders for farther information.
You weren't. If you really want in then you would respect the coven's decision and wait. Age limits are not for restricting knowledge but to help out the well being of a coven. Some covens do make exceptions for members whom are under the age limit have been proven worthy.