There have been some other young Christines in Phantom. One who stands out is Samantha Hill, who started as the alternate and quickly went on to be the main Christine. I don't know her exact age, but she looked VERY young when I saw her as Christine in November 2012. When she left the show, she went to Toronto to star in the new Les Miz tour, although I don't remember whether she was Fantine or Cosette. The other young woman that I thought was fantastic in the role of Christine is Mary Michael Patterson. She, too, seemed very young.
Audrey
Audrey, the Phantom Phanatic, who nonetheless would rather be Jean Valjean, who knew how to make lemonade out of lemons.