An excellent translation of this saying is found in the English translation of Don Quixote by Edith Grossman (Harper Collins, 2003), page 477. She captures the meaning perfectly: "The sun has not yet gone down," said Don Quixote, "and as Sancho grows older, with the experience granted by his years he will be more skilled and more capable of being a governor than he is now."
"see above for the complete sentence"
