Suppose a man dies before the age of thirty-five. On reincarnating, those forces develop in him which he did not use in the years which would have followed his thirty-fifth year. Such men, who before the thirty-fifth year go through death in this way in one incarnation, thereby economise for the next incarnation certain forces, which would have been exhausted if they had lived till fifty, sixty or seventy years of age. The forces which they thus saved are added to those with which they incarnate in the next life. Thereby such souls are born into bodies through which they are in a position, especially in their youth, to confront life with strong impressions. In other words: when such souls, who in their last incarnation died before the thirty-fifth year, reincarnate again, everything makes a strong impression on them. They are deeply stirred by things, they enjoy things deeply, they have living feelings and are easily urged to impulses of will. They are those who take a strong position in life, and who receive a mission. A man does not die without cause before his thirty-fifth year; he will thereby receive a quite definite mission in his next life. These things are complex, and death before the age of thirty-five may also bring about other things — it is not absolute law, for these are only examples. […]
Now, if an especially active man is summoned from life at an early age, it would then occur that in his next life his Karma would appoint him to a quite definite task in life, which he would certainly accomplish. He would be as if predestined. If, however, a man is especially active in one life and lives to a good old age, these forces are then intensified inwardly. He has then in his next life a more complicated task. Outer activity withdraws, and there appears in the soul the necessity to evolve inner activity.
Source: Rudolf Steiner – GA 157a – The Forming of Destiny and Life after Death: LECTURE 2: ON THE FORMING OF DESTINY – Berlin, 18th November, 1915.