On the final question of how to invest to prepare for the proximity of death in old age, Cicero is not very helpful. (Who is?) He offers his "philosophical" view. Death brings at least an end to those troublesome appetites and may open the way to something better--he doesn't say what. Perhaps one could indoctrinate oneself in that attitude in youth and so prepare for old age. But obviously Cicero did not find that philosophy sufficient. As the introduction to the Loeb edition says, "In February 45 [a year before he wrote "De Senectute"] the death of his adored and only daughter drove him into a frenzy of writing in an effort to forget his grief." He did not accept the fact of his daughter Tullia's death with the thought that she had at least escaped all appetites. His philosophy might have prepared him for his own death; it did not prepare him for the death of one he loved.
Later he wrote an essay titled "Consolatio," which probably dealt with this subject and which I have not read. If he was rescued from his grief, it was apparently time and work, not philosophy, that rescued him.
: Herbert Stein thanks his high-school Latin teacher.