Hmmm, you are right. She was in her fifties at the time of the sinking. Maybe it was her personality and interests that inspired the character of Rose, not her appearance or age. Ms. Candee was a very forward thinking and very well traveled feminist who had some interests that would have been considered unconventional for an Edwardian socialite. From what we can gather of Rose's later life in the movie, she and Ms. Candee would have had a lot in common.