A nonagenarian is a person in their nineties.
'Nonagenarian' is the correct spelling (not 'Nonogenarian').
octogenarian Someone who is between 80 and 89 years old is called an octogenarian. If that's not what you're looking for, senior, older person, or some similar term may work.
Frank Moody Mills has written: 'Early days in a college town' -- subject(s): Wabash College, Crawfordsville, Ind 'The notings of a nonogenarian' -- subject(s): Longevity, Old age