Hello Brownian Movement,
at least 2 musicians come to my mind who were avantgarde over a long time of their lives resp. were avantgarde multiple times:
1) Arnold Schoenberg - he again and again changed his cutting edge style until he reached his 12 tone system at an age of 47.
2) Miles Davis: "Birth Of The Cool"; "Kind Of Blue" - modal playing; 60's quintet; around 70: "Fusion". All these points in his life were avantgardistic.
"When you don't know what to do, then do nothin'" (Miles Davis)