What is a ninety year old called?

A 90-year-old is called a "nonogenarian". An 80-year-old is called an "octogenarian", and a person who is 100-years old is a "centenarian".

I call them old ;)

In what capacity? Could just call them a ninety year old. Or senior citizen. If they are related to you, probably better to name them in the capacity that they are related to you. Rarely do people enjoy having their age flaunted.

Depends.

nonogenarian

a bad ass, grey panther, et al. "you go!" senior citizens!

Naked man, 91, holds drunken intruder at gunpoint

Sun Sep 20, 2009/ 2:28 pm ET

LAKE WORTH, Fla. – Authorities said a naked 91-year-old man was able to hold a drunken intruder at gunpoint until deputies arrived. Robert E. Thompson jumped out of bed early Saturday when his dog starting growling and attacking the intruder. He said he got his revolver and went out back to let the guy know how he felt about home invaders.

Thompson, a World War II veteran, said he didn't even notice he was standing outside in the dark without any clothes on.

Authorities arrived to find a drunk intruder being held outside by a naked homeowner.

The 26-year-old intruder was booked into the Palm Beach County Jail on a burglary charge.

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Information from: The Palm Beach Post, http://www.pbpost.com

Joan Rivers
http://blog.foreignpolicy.com/files/images/joan_rivers.jpg

headed for old age

As soon as a person turns 90 you may call them a nonagenarian. They will remain a nonagenarian until they turn 100. Then you can call them a centenarian.
Please note that the spelling of nonagenarian is incorrect in one answer.
The word nonagenarian comes from Latin nonagenarius which means "containing ninety".