Well steroids were made by Germany in 1935, Testosterone. Got introduced to sports by way of the olympics in the 50***8217;s so I would be willing to bet they weren***8217;t readily available for a long time.
When it was invented they were using it as a therapeutic for multiple things so that would be when trt started. As for it going mainstream, I***8217;m not sure about.
Steroids went mainstream in the 80***8217;s. Sure Arnold and the like we***8217;re usind way before but they become readily available to the average Joe in the 80***8217;s.