I googled images of Lea Salonga from Cinderella. Honestly she doesn't look that old as Cinderella. Granted it was 5 years ago, but I don't see her age as a huge issue as everyone says it is.
That being said, to answer the original poster's question, just because someone has a history with being in a show doesn't mean they'll be called back to do the role. There are many factors to consider, including availability.
"We like to snark around here. Sometimes we actually talk about theater...but we try not to let that get in our way." - dramamama611