Seldom does an individual enter upon highly creative effort in any field of endeavor before the age of forty. The average man reaches the period of his greatest capacity to create between forty and sixty

... It is true — certainly in mathematics; what I say now will not apply to the School of Historical Studies where people tend to make their main contributions at a much later age — in mathematics and physics, the prime period in one's life in probably over by, say, 45 or so. That's a bit conservative. Some might put it a bit earlier.
Of course one can continue to work, and do very good work afterwards, but certainly the most productive period would be before that, between 25 and 45.

If you cannot drop a wrong problem, then the first time you meet one you will be stuck with it for the rest of your career. Einstein was tremendously creative in his early years, but once he began, in midlife, the search for a unified theory, he spent the rest of his life on it and had about nothing to show for all the effort. I have seen this many times while watching how science is done. It is most likely to happen to the very creative people; their previous successes convince them they can solve any problem, but there are other reasons besides overconfidence why, in many fields, sterility sets in with advancing age. Managing a creative career is not an easy task, or else it would often be done. In mathematics, theoretical physics, and astrophysics, age seems to be a handicap (all characterized by high, raw creativity), while in music composition, literature, and statesmanship, age and experience seem to be an asset. As valued by Bell Telephone Laboratories in the late 1970s, the first 15 years of my career included all they listed, and for my second 15 years they listed nothing I was very closely associated with! Yes, in my areas the really great things are generally done while the person is young, much as in athletics, and in old age you can turn to coaching (teaching), as I have done. Of course, I do not know your field of expertise to say what effect age will have, but I suspect really great things will be realized fairly young, though it may take years to get them into practice. My advice is if you want to do significant things, now is the time to start thinking (if you have not already done so) and not wait until it is the proper moment — which may never arrive!

Take the sum of human achievement in action, in science, in art, in literature—subtract the work of the men above forty, and while we should miss great treasures, even priceless treasures, we would practically be where we are today … The effective, moving, vitalizing work of the world is done between the ages of twenty-five and forty.

Those who succeed in an outstanding way seldom do so before the age of 40. More often, they do not strike their real pace until they are well beyond the age of 50.

One is rarely an impulsive innovator after the age of sixty, but one can still be a very fine orderly and inventive thinker. One rarely procreates children at that age, but one is all the more skilled at educating those who have already been procreated, and education is procreation of another kind.

I would suggest... the idea of a work which should have for its object the analytic examination of the development of our intellectual faculties for each age. Now, I have aimed to present, in the work here reproduced, only an essay, only a particular example, of such an analysis, "which tends to show that the maximum of energy of the passions occurs about the age of twenty-five." The minimum is not then determined; and even when it shall be, by a sufficient number of observations, one will no more be able to apply it to any given individual in particular, than one could make use of a table of mortality to determine the period of his decease.

the most productive people are the ones who strike a balance between the two extremes, who understand the power of capturing and organizing what they have to get done, but who also don’t sacrifice real work in favor of being productive about productivity.

At the age of ten, twenty, a hundred, very young, a little older, and very old, an artist is always an artist. Isn't he better at some times, some moments, than at others? Never impeccable, since he is a living, human being?

The most productive people work at a pace somewhere between the monk and the stock trader — fast enough to get everything done, and slowly enough so they can identify what’s important and then work deliberately and with intention.

Statistically, it would seem improbable that any mathematician or scientist, at the age of 66, would be able through continued research efforts, to add much to his or her previous achievements. However I am still making the effort and it is conceivable that with the gap period of about 25 years of partially deluded thinking providing a sort of vacation my situation may be atypical. Thus I have hopes of being able to achieve something of value through my current studies or with any new ideas that come in the future.

When someone tells you about the peak year of human history, the period of time when things were good before things went downhill, it will always be the year when they were 10 years old—which is the peak of any human’s existence. Factor that into what they say.

There is a saying that great genius matures late. If something is not brought to fruition over a period of twenty to thirty years, it will not be of great merit. When a retainer is of a mind to do his work hurriedly, he will intrude upon the work of others and will be said to be young but able. He will become over-enthusiastic and will be considered rather rude.

Some of the most productive times in the histories of nations have been when they were badly stressed — economically, politically, culturally or socially. It's possible to be stressed to a point that more creativity is stimulated than would otherwise be the case. I think it is true that necessity is the mother of invention.