What causes hair to grey early?

Mainly genetics, plus oxidative stress, smoking, vitamin B12 deficiency, thyroid disease, and rarely vitiligo or pernicious anaemia. Once melanocyte function is lost, greying is generally not reversible.

Genetics is the dominant factor, and greying from the mid-twenties is common in Indian families.

Worth checking vitamin B12, ferritin and TSH, since deficiency-related greying is one of the few potentially reversible causes if caught early.