What makes one grow old?

It is not the number of years you have lived that makes you grow old. You become old when you stop progressing.
As soon as you feel you have done what you had to do, as soon as you think you know what you ought to know, as soon as you want to sit and enjoy the results of your effort, with the feeling you have worked enough in life, then at once you become old and begin to decline.
When, on the contrary, you are convinced that what you know is nothing compared to all which remains to be known, when you feel that what you have done is just the starting-point of what remains to be done, when you see the future like an attractive sun shining with the innumerable possibilities yet to be achieved, then you are young, however many are the years you have passed upon earth, young and rich with all the realisations of tomorrow.
 And if you do not want your body to fail you, avoid wasting your energies in useless agitation. Whatever you do, do it in a quiet and composed poise. In peace and silence is the greatest strength.
 If the growth of consciousness were considered as the principal goal of life, many difficulties would find their solution.
The best way of not becoming old is to make progress the goal of our life.
[CWM2, 12:123]

How to remain young

To be young is to live in the future.
To be young is to be always ready to give up what we are in order to become what we must be.
To be young is never to accept the irreparable.
[CWM2, 12:122]
Youth does not depend on the small number of years one has lived, but on the capacity to grow and progress. To grow is to increase one’s potentialities, one’s capacities; to progress is to make constantly more perfect the capacities that one already possesses. Old age does not come from a great number of years but from the incapacity or the refusal to continue to grow and progress. I have known old people of twenty and young people of seventy. As soon as one wants to settle down in life and reap the benefits of one’s past efforts, as soon as one thinks that one has done what one had to do and accomplished what one had to accomplish, in short, as soon as one ceases to progress, to advance along the road of perfection, one is sure to fall back and become old.
[CWM2, 12:257]
There are young people who are old and there are old people who are young. If you carry in you this flame for progress and transformation, if you are ready to leave everything behind so that you may advance with an alert step, if you are always open to a new progress, a new improvement, a new transformation, then you are eternally young.
[CWM2, 3:238]
The definition of youth: we can say that youth is constant growth and perpetual progress—and the growth of capacities, possibilities, of the field of action and range of consciousness, and progress in the working out of details.
[CWM2, 8:20]