Mature in what context? Practicing what exactly?
If I was going to try for something generic — I'd say that characteristics I'd be looking for would be a rejection of the idea that practices can be "mature".
We should always be looking for ways to grow. We should always be looking for alternatives to our existing techniques and different and more effective ways of doing them. We should be trying to figure out new techniques that work better in specific contexts.
I'm still learning. I'm doing things differently now than I was two years ago, five years ago, ten years ago, twenty five years ago. That's because I'm learning new things, working in different ways, with different things, with different products, with different people, with different balances of skills, in different contexts, on different kind of projects.
A mature practice doesn't have a fixed set of skills. It has a pattern language of skills and practices that they can use to figure out the right thing to do in the right context. They also have a firm grasp of the fact that the pattern language is a work-in-progress that will never be completed because the world we live and work in isn't a static place.
Mature sounds too darn static to me. We grow or we die.