وَبِالْمَشِيبِ إِنْذَارُكَ فَمَا أَعْذَارُكَ؟
And greying hair is your warning; what excuses do you have?
Mashīb refers to hair turning grey; indhār is an advance warning, and aʿdhār is the plural of ʿudhr, an excuse. A visible sign of passing years becomes an occasion for inner reflection.
This is a literary and moral sign, not a medical claim that everyone greys at the same age. The speaker’s concern is diminishing opportunity.