The reason why wages in China are rising so fast is not because of a aging population. But its because the economy is moving up the value chain and is also diversifying away from (low end) manufacturing. It is becoming more difficult for low end manufacturers to find workers. As people in China no longer have to go to the coastal cities to work in city sized factories as there are oppertunities closer to home. The ones that are moving out of China are the lower end manufacturers that are facing higher labor costs and more stringent environmental rules. But where China is losing out on the lower end China is gaining in the mid to higher end. You can see that in the fact that China every year is gaining a ever larger share of the world's industrial output as well as a larger share of the global export market.
This is true, quite a bit of manufacturing are moving interior or westward. The wages there are cheaper and the trained migrant workers are usually the pioneers that brings work back home. They would first start getting contract on producing part of the products and over the year, they would be building the entire product. The original boss from the coastal cities could shut down the manufacturing operation and contract the migrant worker to produce the product from their home town. As second and third tier city grows, manufacturing also get shifted there since stable infrastructure would be build.