Screwtape advises Wormwood on using time to wear down a soul:
The Enemy has guarded him from you through the first great wave of temptations. But, if only he can be kept alive, you have time itself for your ally. The long, dull, monotonous years of middle-aged prosperity or middle-aged adversity are excellent campaigning weather. You see, it is so hard for these creatures to persevere. The routine of adversity, the gradual decay of youthful loves and youthful hopes, the quiet despair (hardly felt as pain) of ever overcoming the chronic temptations with which we have again and again defeated them, the drabness which we create in their lives and the inarticulate resentment with which we teach them to respond to it—all this provides admirable opportunities of wearing out a soul by attrition. If, on the other hand, the middle years prove prosperous, our position is even stronger. Prosperity knits a man to the World. He feels that he is ‘finding his place in it’, while really it is finding its place in him. His increasing reputation, his widening circle of acquaintances, his sense of importance, the growing pressure of absorbing and agreeable work, build up in him a sense of being really at home in earth, which is just what we want. You will notice that the young are generally less unwilling to die than the middle-aged and the old.
From The Screwtape Letters
Compiled in A Year with C.S. Lewis