Yep, I would say it started with Denny -- his first JLA story (#66 in 1968) had a political theme -- unusual for the straight-laced non-personality team. The old-guard creators were replaced by Adams, Ditko, Giordano, Aparo, Robbins, Novick, Friedrich and eventually Kirby etc.

Basically from 1968-70 (as Lee says), the same period I consider the Bronze Age phase in.