The numbers are not arbitrary. They are used to measure how long a person has been alive, which is kind of statistically significant, and yes, largely correlates with maturity. I'm not 26 mature points, I'm 26 years old.
That's where we disagree. You say that as if it were a proven fact. If you got studies on that, please report.
My point is, that at least from my experience there is a lot more to maturity than mere age. And you can't really know if you just superficially look at someone and their age.