The notion that 'life begins at forty' is a 20th century one; prior to that it was more accurate to say 'death begins at forty' as most people didn't live much beyond that age. Life expectancy in mediaeval England was around 25 years and only reached forty sometime around the turn of the 20th century. By the 1930s many, in western societies at least, could expect a decent spell of reasonably affluent retirement, free from work and the responsibilities of childcare. Household gadgets like washing machines and vacuum cleaners were becoming more widely used and had begun to relieve women's drudgery and offer them increasing amounts of leisure time as compared to their Victorian mothers.

In 1932, the American psychologist Walter Pitkin published the self-help book Life Begins at Forty. Pitkin stated confidently:

Life begins at forty. This is the revolutionary outcome of our New Era. Today it is half a truth. Tomorrow it will be an axiom.

Pitkin is often credited with coining the phrase and, while it is true that his popular book was the cause of it becoming part of the language, he wasn't the first to express the idea, or even the phrase itself. The take-up of the idea was rapid and 'life begins at forty' appears many times in newspapers and other printed records from 1932 onwards. This was propelled further into the American consciousness in 1937 via a recording of the song 'Life begins at Forty', written by Yellen and Shapiro and sung by Sophie Tucker.

However, we need to go back a way to find the origin of the phrase. The great 19th century German philosopher Arthur Schopenhauer came close to it with his view:

"The first forty years of life give us the text: the next thirty supply the commentary."

In keeping with the reduction of the toil of domestic work and child rearing that began freeing up women's free time to some extent at the start of the 20th century, the first reference to life beginning at forty refers specifically to women.

Mrs. Theodore Parsons was Physical Director of Schools in Chicago and, in 1912, wrote Brain Culture Through Scientific Body Building. It wasn't a runaway best-seller, but the arrival of the USA in the First World War in 1917 gave her views a new lease of life. In April of that year The Pittsburgh Press printed a feature on Mrs. Parsons and her no-nonsense opinions about the benefits of a brisk exercise programme that she acquired from her soldier husband (Mr. Theodore Parsons was, sadly, recently deceased - presumably from exhaustion):

"The average woman does not know how to breathe, sit, stand or walk. Now I want women to train for the special duties which may devolve upon them in war time. Death begins at thirty, that is, deterioration of the muscle cells sets in. Attention to diet and exercise would enable men and women to live a great deal longer than they do today. The best part of a woman's life begins at forty."

What special duties Mrs. Parsons had in mind, stationed as she was in Chicago, isn't clear. The paper was good enough to include a graphic of the dynamic couple, demonstrating their method in action, so you can give it a try and see if it works.

Life expectancy has continued to move on and forty now seems no age at all. In 1991, the New York Times printed this opinion:

All our age benchmarks, which used to seem solid as rocks, have turned into shifting sands. 'Life begins at 40? More like 60'.

http://www.phrases.org.uk/meanings/life-begins-at-forty.html

Another explanation for this phrase by the free dictionary online:

Life begins at forty: Prov. By the time you are forty years old, you have enough experience and skill to do what you want to do with your life. (Often said as an encouragement to those reaching middle age.)

Alan: Why are you so depressed? Jane: Tomorrow's my fortieth birthday. Alan: Cheer up! Life begins at forty. For Pete, life began at forty, because by that time he had enough financial security to enjoy himself now and then, rather than having to work all the time.

这与汉语中的“四十不惑”有异曲同工之妙。

人生路途，四十年算是第一场彻悟：

见证了生命的神奇、生活的艰辛与欢乐、工作的压力与解脱、亲情友情爱情、生老病死、悲欢离合......

岁月如白驹过隙、往事成过眼烟云、顿悟了舍得与豁达、理解了宽容与洒脱。

心智成熟犹如家蚕吐丝，历经沉默与修炼，再脱颖而出；

凤凰涅磐之后，无需徘徊、也不再随波逐流。

估计这才是人生乐趣四十始（Life begins at forty)的真谛！

愿天底下所有为人生与事业打拼的朋友们，都能拥有一份如斯的平静与坦然。