What was Winston’s age when his mother died?

Winston was a child when his mother disappeared. He is not sure how old he was, but he thinks he was 10 or 11.
At one point, Winston also thinks:

His mother’s death, nearly thirty years ago, had been tragic and sorrowful in a way that was no longer possible.

Later, having fallen in love with Julia, he more clearly remembers the circumstances surrounding his mother's disappearance. He recalls he was at least 10 but no more than 12 when he last saw her. He was hungry all the time and stole a piece of chocolate from his starving little sister—then ran out of the room where they lived. When he came back, his mother and sister were gone. At this point, he backtracks on the idea that his mother is dead, thinking:

To this day he did not know with any certainty that his mother was dead. It was perfectly possible that she had merely been sent to a forced-labour camp.

After his arrest, he meets a drunken old woman in detention who says her name is Smith, and he notes that she might be his mother. While unlikely, it is not impossible:

She might, thought Winston, be his mother. She was about the right age and physique, and it was probable that people changed somewhat after twenty years in a forced-labour camp.

The key points are as follows: first, he sees his mother for the last time when he is about the 11. Second, while he does not know if she is dead or alive, she is gone from his life thereafter. Third, she is woven deeply into the fabric of his soul, especially in her love him. This is the most important point of all. O'Brien can get to Julia—he knows Winston loves her—but he never penetrates fully to Winston's mother. It is significant that at the end of the book, Winston is thinking of a very happy moment playing Snakes and Ladders with his mother shortly before he dies. Did the state get full control of him, or did a part of him elude its grasp?