I think check user generates certain legal risk to the Foundation especially when he is a minor. Wikimedia Foundation has a privacy policy. It seems the Foundation is expressively promising that certain information will not be released to the third party unless specific conditions are met. And here, "third party release" does not include, at least the way I read the privacy policy, release of personal information from Wikimedia Foundation to a check user. It suggests that, at least in the context of privacy policy, the check users are insiders for the Foundation, not a third party. This, in turn, means that the Foundation has a legal responsibility to make check users to understand and follow its privacy policy. So when check user breaks the promise - i.e. violate the Foundation's privacy policy, one may question if the Foundation is partly responsible for the violation. If a check user is legally a minor, he may be able to legally get away with breaking promises he has made, including the compliance with privacy policy. I am not sure if minors really are less reliable than adults, but if they are equally unreliable, then the Foundation is more responsible for minors' violation of privacy policy than adults. So, not because minors are less reliable, but because adults can bear more legal risk when they abuse their check user privilege, it is legally safer for the Foundation to limit the check user to adults. How significant this difference? That is perhaps open to debate. I personally think that the better course of action to mitigate the legal risk is to treat check users as outsiders in the privacy policy. I am not a lawyer, so be reminded that my reasoning could be flawed.. Best, Tomos