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.
His only daughter died, which led him to write as a distraction for his grief.
His account of coping with the death of his daughter was not recorded, but is said to have been noted during an interview.