In European improvised music, I really think of things as getting underway in the 1970s - though certainly Brotzmann, Wilen (who seemed to embrace "free jazz" around 1965-1966), Stevens, Schoof, von Schlippenbach, Tusques and others were on the scene before that.
Cage, sure. Funny, my mind usually thinks of his work tied to the 1940s and 1950s. With Reich, certainly he began his explorations in the 1960s but the 70s was really his "decade." Same with Glass. Maybe Riley too, though I know he was quite advanced with his work in the early '60s along with LaMonte and Terry Jennings.