Septuagenarian, octogenarian, ....

Larry asked about a word for a person who is ninety+ years of age.

As you may know, the word for a person in their seventies (70's) is septuagenarian.

The word for a person in their eighties (80's) is octogenarian.

And the word for a person in their nineties (90's) has the same ending as the other two: nonagenarian.

Here is a sample sentence with nonagenarian:

• Last year, when my neighbor became a nonagenarian, she decided she was too old to live by herself, and moved in with her daughter.

I hope this helps.