In your nineties
Definition 1 of 1
Pro Tip 1/3Age Specificity
Used when referring to an age range, not exact age. Suggests approximation within the nineties. 
She's in her nineties, closer to a century old than to eighty.
Pro Tip 2/3Contextual Use
Often implies respect or admiration, especially when talking about someone's abilities or health at such an advanced age. 
At 95, he still solves crosswords daily, sharp in his nineties.
Pro Tip 3/3Colloquial Flexibility
More commonly used in casual speech, rather than in formal writing or documents. 
My neighbor is in her nineties but chats away like a teenager.