10-11-12 would probably be a good age. There are several good restaurants, and about an equal number of Asian grocery stores worthy of browsing if just to see the different kinds of food that you will never see in a Shop Rite or Walmart. Also a few other Asian businesses like travel agents, beauty salons catering to Asian tsastes. It's fairly small compared to NY, about 4 blocks by four blocks. One of the streets has a big carved arch over the street known as the Friendship Gate. It's a visually stimulating and aroma filled place, educational at the least. My wife is Asian, so we always stop in there when in Philly, for the food, especially groceries that we can take home. Asian candies are quite different than the typical American stuff if your kids are adventurous enough to try flavors like mango, and papaya.
A little more info and some images here:
http://en.wikipedia.org/wiki/Chinatown,_Philadelphia