"... it refers to gray hair"

"Did not We grant you a life long enough for him who reflected to reflect therein? And there came to you the warner". [35:37].

It's narrated in "Tafsir Bahr Al-Muheet" regarding that Ayah:

وقيل : النذير : الشيب ، قاله ابن عباس

ENGLISH:

"And it was said: "the warner", refers to gray hair. This was the saying of Ibn Abbaas".

NEDERLANDS:

"En er werd gezegd: "een waarschuwer", verwijst naar grijs haar. Dat was de uitleg van Ibn Abbaas".

["Bahr Al-Muheet", 7/317].