The bar is set, because there needs to be a bar. 18 seems to high. 12 too low. Children may love to GO for a drive, but they need to be about 16 in order TO drive, in most US states. There still are (in some places) rites-of-passage that mark entry into the adult world. Debutante-balls are common in the US South.
The standard is set, much like we insist on church membership and adherence to a recognized, Reformed church-confession. How old does one have to be to really be able to recognize a "confessional commitment"? We have a "baseline" clientele. A person needs to have grammar and logic behind him, because this is a rhetorical site. Obviously, there are adults for whom the age "standard" is nothing but a box-to-check; the conversation is still a challenge, but we can expect most of them should have a modicum of maturity. When we admit our children to the Lord's Table, we think they should have been catechized, and have a baseline theological commitment. Most of those exams take place in the mid-to-late teens. Thus, our standard roughly comports.
But still, we have to police the maturity level all the time. Add children to the situation, and that's too much work. We also expect people to argue here--civilly, but we expect a certain amount of disagreement and friction. Can a child handle an adult "rebuke"? How is a person, who means to be blunt (but fair), supposed to "pull his punches" dealing with a (potentially) fragile ego, particularly when he expects a person to be at least 16? The fact that a few 11yr olds may have the requisite character to contribute to the board as active members doesn't mean that they are therefore qualified. Experience is a greater factor in maturity than brain power.
This board is designed as a place for learning. It is possible to learn a lot as a listener--more, in fact--than when speaking, which is exactly the manner a young person can best use this board. Reading will prepare a young person for active engagement with grown-men and women, when once that "official" mark has been passed.
We also do not want to encourage a youth to make this place their primary theological home. That should be in their home church, whenever possible. Face-to-face interaction with a mentor is better than the internet. Ideally, home first, then church, are the front-line for theological inquiry. In the meantime, reading here can fill in; You, Samuel, can help out. You could ask a question on her behalf. But not every 14yr old will have such a filter, so we have to emplace an age cut-off. Nothing personal, nothing against children.
Obviously, we cannot eliminate liars and other fakes--this is an honor system. And I think that a person who waits to meet the standard will be rewarded for his or her patience. If he or she gets angry at the limit, and huffs off, that pretty much proves its worth.