Demystifying Depression/Depression and Ageing
Depression and Ageing edit
You do not see 60-year olds having the same lifestyle as 20-year olds. Even a 40 or a 30-year old probably would not be able to accommodate for a long time all the intense living and partying of their youth. We naturally accept that our physical abilities decrease slowly with age, and our brains are no different. Mind you, in this context I will speak only of the brain's endurance, not of the general cognitive abilities. Therefore, do not interpret the graph in Figure 9 as "getting dumber with age". (Though it is most likely that cognitive abilities also decrease with age. Luckily, the added experience can in large part compensate for that).
Bear in mind that I am largely speculating here, but I would not be surprised if the reason why depression tends to strike first towards the mid 20s (and this is a fact) is related to a dip in the maximum normal capacity which happens after adolescence. Many people simply fail to accommodate for the necessary changes in their lifestyle, and thus find themselves constantly going over their (now slightly diminished) limits.
Still on the speculation front, consider the fact that people tend to sleep less as they get older. Could it be related with the graph in Figure 9? If sleep is indeed fundamental for the brain to repair itself, and if age cuts down the requirements for the maximum normal capacity, it is not too far-fetched to imagine that people would therefore require less sleep as they get older.
Speculations aside, do not look with gloom at the graph. Ageing is not a death sentence as far as feeling well is concerned. People do generally accommodate by making changes to their lifestyle, and remember that depression only arises should you constantly go over your limits. Furthermore, in percentage terms, the natural decrease might not even be large. (Unfortunately, our current understanding of depression does not yet allow us to make precise quantifications. See the section Quantifying Depression for details).