«أَشَابَ الصَّغِيرَ ... كَرُّ الْغَدَاةِ»
The recurrence of morning made the young grow gray.
أَشَابَ: past-tense verb, “caused to become gray-haired.”
الصَّغِيرَ: its accusative object.
كَرُّ: nominative grammatical subject, with «الغداة» as its genitive complement.
Grammatically “recurrence” is the subject, but rhetorically the point is that it is not the literal independent agent of the effect. Syntactic agency and the deeper causal attribution need not coincide.