As to when it actually began, that's not a bad question but difficult to put a single date on because there are so many different components. Also it has to wait until someone notices it, which comes after it's actually underway. Looking at environmental collapse for instance, Rachel Carson noticed it underway in the 60's, but it certainly began before that. It only really becomes visible in retrospect, and there has to be some time interval to see develop.
Since oil is such a big part of the energy aspect, you could peg the beginning to when the 2nd derivative of new discoveries turned negative. That would put it in the mid 1960s.
Another way to look at it would be the 2nd derivative of global population. That peaked in 1962.
Just like a car that breaks down doesn't immediately stop or roll backwards, it begins to slow down first (in most situations), same is true for civilization collapse. Picking out the moment it began slowing can be tough as it's almost imperceptible. As it progresses it becomes more obvious. Now with industrial civilization it has become quite apparent, and has been so since the early 2000s for any who is not so blind they will not see.
RE