Yes, fine, sure, age does not equal maturity. But its a damn good indicator.
And please note, when I say maturity, I dont just mean they act respectful. Furthermore immature and stupid are two totally different problems, and we often end up banning people for doing stupid things, and so thats mucking up all the data, because there are stupid people of all ages. Immaturity is not the only ban reason by far.
But more importantly, when we say mature, we mean it in the "This film contains mature content" sense. Not just general respect, but also an understanding and preparedness to deal with ideas and concepts that are of a more adult nature. Obviously nothing explicit, but romance has always been allowed by the server but its hard to do when you arent sure if the person on the other side of the screen is 12 or 18, and that makes a huge difference. Or even just the implied gore/torture aspect, which is allowed assuming you dont take it too far. First of all, we dont want to expose 11 year olds to things like, for example, what Scardracs character does, and secondly, we dont think they can contribute positively to that sort of world.
16 is a relatively arbitrary number however, and that number was not picked through much thought. As such Im totally fine moving it down to 15. Id be reluctant to go any below that.