To answer the original question, you politely correct her the same way you might correct a younger person. If, as you say, she is perfectly in her right mind, then you correct her at 90 the same way you corrected her at 60 - nonchalantly, quietly, nonconfrontationally. "Actually, Grandma, I think that the 7 million figure was for the whole country of China, but I agree that is quite a bit either way!" Or, "Is it really 7 million, that sounds so high! Let me check the article...oh yes, here it is, 7 million students in all of China..."
I think what many other posters are reacting to, though, is the notion that a 90 year old woman should care what other people think about her, especially with respect to something so innocuous as getting a figure wrong. Unless she makes her money doing complex calculations, I also can't see what the big deal is. At that age, I think she's earned the right to be (very slightly) eccentric!