Why do we measure experience in years?

Many years ago a friend and I were discussing a rather pompous mutual acquaintance who was always referring to the fact that he had thirty years experience in (whatever it was - I forget).

My friend asked "Has he got 30 years experience or has he got one year's experience 30 times?"

Once I had got over my initial envy at not being the originator of this amusing insight, I started thinking. Recruitment processes typically ask for, say, five years experience in a particular area. But most people recognise that learning does not simply increase proportionally with time. The ubiquitous "steep learning curve" that many of us anticipate travelling up indicates this: 

The amount we learn in the last of five years is unlikely be the same as we learn in the first of five years. It is probably going to be considerably less. Certainly from my own experience, I would estimate I learn most in the first 3-6 months.

So what does "five years experience" really mean? And how many years is it before the increasing number starts to have a negative connotation? Is five years in the same role two years too many?

Edgar Bolton 2013