Here's a good article on how age progression is done at the Forensic Anthropology and Computer Enhancement Services Laboratory. From the article:
A child's face broadens and lengthens, because faces grow downward and outward. Secondary teeth grow in, and the bridge of the nose rises. The cranium expands, the eyes narrow, the mouth widens and the nose lengthens. Light-colored hair tends to darken. By age twelve, the face looks fairly mature, with the chin forming and the nose still growing. Eventually the cheekbones take on more prominence and eyebrows fill in.
I'm no expert, but I'd imagine that the apps you can find online and at Apple's app store probably take common tendencies like these into account and apply them to any photo you upload. It seems like a lot more information is required for accurate results.