From The Washington Post:
As a 42-year-old man born in England, I can expect to live for about another 38 years. In other words, I can no longer claim to be young. I am, without doubt, middle-aged. To some people that is a depressing realization. We are used to dismissing our fifth and sixth decades as a negative chapter in our lives, perhaps even a cause for crisis. But recent scientific findings have shown just how important middle age is for every one of us, and how crucial it has been to the success of our species. Middle age is not just about wrinkles and worry. It is not about getting old. It is an ancient, pivotal episode in the human life span, preprogrammed into us by natural selection, an exceptional characteristic of an exceptional species.
Compared with other animals, humans have a very unusual pattern to our lives. We take a very long time to grow up, we are long-lived, and most of us stop reproducing halfway through our life span. A few other species have some elements of this pattern, but only humans have distorted the course of their lives in such a dramatic way. Most of that distortion is caused by the evolution of middle age, which adds two decades that most other animals simply do not get. An important clue that middle age isn’t just the start of a downward spiral is that it does not bear the hallmarks of general, passive decline. Most body systems deteriorate very little during this stage of life. Those that do, deteriorate in ways that are very distinctive, are rarely seen in other species and are often abrupt. Each of these changes can be explained in evolutionary terms. In general, it makes sense to invest in the repair and maintenance only of body systems that deliver an immediate fitness benefit — that is, those that help to propagate your genes. As people get older, they no longer need spectacular visual acuity or mate-attracting, unblemished skin. Yet they do need their brains, and that is why we still invest heavily in them during middle age.