Of course, but change doesn't necessarily imply arbitrariness, especially not when you're talking about the process of understanding natural phenomena like brain- and personality development.
It implies arbitrariness when you look at the cultural standards of employment over the last two hundred years. It goes from age 8 all the way to 18. What it implies is that people really have no idea what the heck they are doing, and they barely know what they even know, let alone what they don't. My initial point was that the age at which a person becomes a legal adult is not reflective of some kind of universally understood maturity.
A practice effect could explain that.
It could explain that but you are basing your responses off of self-theorization and guesswork. If you look at actual causes for crashes and documented neuromotor development of teenagers, their reaction time and ability to make the correct logical decisions the quickest is significantly, drastically lower. This is usually the number one cause for accidents. Everything else is as you said a product of individuality. But if you bring individuality into it, you don't really have an argument at all, do you? You just have a bunch of people, lol.
Is it a coincidence that this legal competence thing overlaps with the legal ages of 18 and 21?
A child can be declared legally competent to stand as a witness in trial or even be a defendant, of which there are many documented cases (controversial though they are, because most people tend to say that "anyone under the age of 18 cannot be tried as an adult", because, of course, the Legal Age Of Adulthood). It is up to the individual jurisdiction to decide each and every one of these cases. There was recently a child charged with murder, there's a thread about it somewhere around here.
My original point, as it was stated, is that legal age seems to encompass a socially recognized advent of "maturity" upon suddenly reaching this "miracle number" of 18. But, as is clearly noted, maturity is not often related and is a different concept to adulthood. In my opinion, "adult" is a legal term and maturity and responsibility are what that legal term attempts to measure.