Seeing the future

I finally saw the third Harry Potter movie (Prisoner of Azkaban) this weekend. One thing that I noticed was how different the actors look from how they used to. How do they do casting for these sorts of long-term projects with child actors? Does it involve the same computer technology that is used in the creation of photos of missing children? Or something more advanced that predicts how a person's appearance and voice will change? Which use has driven technological progress?

A quick google search didn't turn up any answers. Maybe somebody out there knows something.