At first I thought it meant she was slightly over fifty years old (fifty years five months), but then I started to wonder if she was in her early (51-53) or in her late (57-59) fifties.
I’ve encountered it in a book written by American author. A young woman thinks of her father’s secretary as “a former teacher and widow on the upside of her fifties”. I don’t understand if the woman in question is supposed to be fifty years several month years old or, for example, she is fifty-three years old.
The only thing we can say fairly certainly is that she is over fifty. I would assume, given the sentence, that perhaps he meant "in her late fifties, so perhaps fifty-seven to fifty-nine.