Obviously there aren't going to be a lot of people who know the exact age, but the fact that she died from illness at a young age is common knowledge. The reason for this is that when learning about Hitler it is considered one of the defining moments of his life that shaped his character. He was reported to have been absolutely devastated by his mother's death as she had coddled him and taken his side against his father who beat him constantly. He had no friends and dropped out of school and she was the only one who supported him.
Her doctor was Jewish and Adolf became convinced that the doctor hadn't done enough to save her.
As historians tend to presume too much, I wouldn't assume that this is entirely accurate, but in terms of being a memorable history lesson, I would think that many people would remember what is considered such a critical detail.