When Zayn Malik dyed his hair grey, I’m reliably informed, sales of grey hair dye rocketed.

Of course, it was all the rage in the 18th century to have grey hair or wigs, as we see in this portrait of Marie Antoinette:

Portrait of Marie Antoniette by François-Hubert Drouais (1781). Image from Wikimedia Commons 

In order to cool down this fashion fervour, let us turn to Rabbi Santob de Carrión (more properly Shem Tov), active in Castile in the reigns of Alfonso XI and Pedro the Cruel. His most famous work in Spanish (he also wrote in Hebrew) is the Proverbios morales, which typically takes an idiosyncratic view of the world.

 
One curiosity of the transmission history of Santob’s work is that it is preserved in Latin script and in Hebrew script. Image from Santob de Carrión, Proverbios Morales. Edited with an introduction by Ig. González Llubera. (Cambridge, 1947). 11453.d.11.

Santob’s contribution to the grey debate runs:

Las mis cañas teñilas,
Non por las aborresçer,
Nin por desdesyrlas,
Nin mancebo paresçer,
Mas con miedo sobejo
De omnes, que buscarian
En my seso de viejo,
E non lo fallarian

[I dyed my hair black
Not to hide my age,
But to stop men thinking
My hair made me a sage.]

Barry Taylor, Curator Romance studies

Reference:

Barry Taylor, ‘Sem Tob de Carrión, Proverbios morales’, in Diccionario filológico de la literatura medieval española: textos y transmisión, ed. Carlos Alvar and José Manuel Lucía Megías, Nueva Biblioteca de Erudición y Crítica, 21 (Madrid, 2002), pp. 941-44. YA.2003.b.1351