What is Middle Age?

I've gone on a research binge to figure out how I am going to think about this whole midlife thing. I should have a pile of books waiting for me at the local library by the end of the week and I dug through some book shelves and pulled out a couple other points of reference. We'll get to those later.
My working hypothesis is that being "middle aged" refers to those years that mark the middle of what would be deemed an average lifespan. Let's be optimistic and say that I'm going to live to 100 years of age. That would make 50 the halfway mark. If I say that there is a middle – then there is also a beginning and end. That smacks of portions divided into thirds – which would make midlife 33 years of age to 66 years of age, right?
Not exactly. It just doesn't seem to work out that way.
So my hypothesis needs to be complicated by other ideas and theories, layered with perspectives, culture and nuances that may or may not help me understand what the hell is going on inside of me. I'm curious to see how others have expressed themselves about this journey. Let's see what they have to say.