In the dazzling years of youth, men do not think that they are immortal, but they cannot imagine themselves dying. The process of aging begins for a man when he realises that he can die, and is probably going to die. In fact this is what is meant by losing one's nerve, the growing consciousness that one is mortal, and the inability to forget it.
In childhood, however, things are different. One looks forward to the years passing, and especially to the time when one will be grown up. But although one looks forward, generally it is little more than day-dreaming, and so far as planned foresight is concerned the young child lives for the moment, and the end of the week or the end of the term at school are generally as far as he can see. In one's teens, one can see a little further, to the scholarship examination to the University a year or two years hence. But one cannot look ahead far, so one looks ahead eagerly, and one does not fear the years passing. Years do pass, but one does not measure time by counting the years which have passed; one is too lost or absorbed in the present moment. One counts time by counting the number of present moments which have passed, whether the moments were pleasant or unpleasant.
Then one arrives! One arrives at adulthood; and one's idea of time changes. It changes so slowly as hardly to be perceptible, but one starts to think of it in terms of experience. The richness of one's life begins to be measured, in one's own eyes, by the number of experiences one has gone through; and therefore one thinks of time as the length of time necessary to contain so many experiences. It is different in childhood, when experiences are things one endures either pleasantly or unpleasantly. One may enjoy them, but one does not think of them as things which enrich one's life. One thinks of them as things which one enjoys. With adulthood a degree of self-consciousness appears, as well as a greater command over one's own destiny.