«وَأَيُّ شَيْءٍ شَيَّبَ لِحْيَتَكَ حَتَّى أَنْكَرْتُ حِلْيَتَكَ؟»
What whitened your beard so that even your appearance seemed unfamiliar to me?
Shayyaba is a Form II verb meaning to cause greyness; liḥyah is the beard; ḥilyah here concerns one’s appearance or recognizable form.
This is not merely a request for an age. Al-Ḥārith asks what life experiences have produced such a striking change. Abū Zayd replies not with plain prose but with poetry, preserving his characteristically literary voice.