Katha-1-1-2-tam̐ ha kumāram̐

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Him, i.e., Nachikôtas, though in the prime of life, i.e., young and not possessed of the power of procreating zeal, i.e., faith in the existence of a future state, entered, induced by the desire of good to his father. At what time is explained; when cows were brought to be distributed among the Ritviks and the sadasyas for their rewards; possessed of zeal, Nachikêtas thus thought.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda