A person in their nineties is commonly referred to as a nonagenarian.

(nonagenarian: someone in their nineties - noun or adjective)

It is harder for a nonagenarian to get a drivers license.

The nonagenarian residents did not know how to use the computerized library.

A nonagenarian is between 90 and 99 years of age. This is preceded by the octogenarian, and is followed by the centenarian.

A 90 year old is called an nonagenarian. Nonagenarian are people who are over the age of 90 years old.

Great-granddad, or more seriously, a nonagenarian