noun, plural nine·ties. a cardinal number, ten times nine. nineties, the numbers, years, degrees, or the like, from 90 through 99, as in referring to numbered streets, indicating the years of a lifetime or of a century, or degrees of temperature: His grandmother is in her nineties. ...

Is it ninety's or nineties?

Strictly speaking, '90s, with the apostrophe, is correct.

Is Ninety a proper noun?

Since ninty is never the correct spelling, remember ninety vs. ninty is easy. Ninety is always the correct way to spell the written form of the number 90. Many style guides recommend using figures instead of words for numbers greater than ten.