IPA: /ˌnɒnəˈdʒɛnəriən/
KK: /nɑnəˈdʒɛnəriən/
A person who is between 90 and 100 years old.
My grandmother is a nonagenarian who still enjoys gardening.
Nonagenarian → It is formed from "nonagenarius" (from Latin, meaning ninety) and the suffix "-an" (meaning relating to or belonging to). The word refers to a person who is in their nineties, thus relating to the age of ninety.
Think of 'ninety' ('nonagenarius') to remember that a nonagenarian is someone who is in their nineties.
No commonly confused words.