What does the prostitute's old age say about Winston and his mother?
Winston's real mother who he has dreams of throughout the book makes Winston feel like she died or was erased because of him. Her life paid for the opportunity for his to remain. That sacrifice (although it is believed her death was symbolic of the great Stalinistic purges) fills him with guilt and affection for her even though his memory of her grows symbolically more faint throughout the book.
Thus, the age of the prostitute (although it is not specifically referred to in the book) might demonstrate a remembrance of his mother. The episode with the prostitute is actually a flashback for Winston, much like a dream. You could say this is an Oedipus Rex moment, but the light was off for most of the scene with the prostitute. He makes it seem as if he didn't understand her age until it was almost over, and then he just finished for the opportunity to have sex. This may demonstrate a longing to have relationship with his mother, but that relationship he wanted was likely appropriate. He may be recalling a time in life when sex was not used to control people, but was used for pleasure.