What an awesome post.
I already had my opinion formed from the title of the article ... and after reading the article, I was convinced (WITHOUT DATA OF COURSE.) I may have a reason this is so ...
When a person is young, they have very little in the way of unhappiness, they are insulated from life's worries, problems and responsibilities most likely due to still living at home or not being on their own too long. A person has not yet had enough time to become cynical.
When a person is older, they have mellowed, let go and accepted the way things are in life. They have come to terms with it all and are at peace with themselves. They are quite happy with their station in life.
BUT IN THE MIDDLE of life ... It is an obstacle course. Everything is a battle. Relationships, children, family, work, financial stress, unbalanced schedule heavily weighted to work vs relaxation ... etc ... How can a person be happy with all this going on in their life?
TO me, it makes a lot of sense that you start out happy and end up happy, with a lot of misery and dissatisfaction in the middle.