A mature player might be reflexively defined as one who is individuated, who has "come to terms" with himself/herself in regard to music. Of course that state of mind is purely internal and might not be communicated to other people. Indeed, a happily "mature" player in this sense might seem three shingles short of a roof to other musicians. You might think of a few names but of course I couldn't possibly comment.
I think what makes a mature performance, is having technique that transcends what you're performing, and then having the innate musicality to do something with what you're playing. I think a mature performer is someone who does this consistently, regardless of repertoire.