From the UK Telegraph story, the University of Warwick’s Oswald explains:
“The first theory is that when you are young you have high aspirations and then in middle age have to learn to quell them. After all we cannot all be captain of the national football team or a rock star.
“The 30s and 40s are therefore painful times when reality sets in but when you get older you’ve learnt to accept yourself.
“The second theory is that people who are happier live longer. Therefore if the unhappy people have died younger then those who remain are happier.
“The third theory is that we learn to count our blessings when we get older. We see friends and family die and we see bad things happen and are just happy to be alive.”
For biomedical scientists, the age 44 has interesting ramifications: the current average age of receipt of one’s first major NIH grant is 42. Perhaps 44 is the age at which one realizes they need to think about submitting their competing renewal application.
In truth, 44 is the age of maximal unhappiness for the British:
[T]he new research discovered that for both British men and women the probability of depression peaked at about 44 years of age. In the US, unhappiness reached a peak at about 40 years of age for women and 50 for men.