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.