Where can I learn to fly in Pandaria?

Once I reach level 90, where do I upgrade my flying skill so that I am able to fly over the island of Pandaria?

Is it possible to learn how to fly over Pandaria before I get to level 90 or is this restricted to max level characters only?

In order to fly in Pandaria, you need to learn "Wisdom of the Four Winds". This can be learned from the flight trainer at your faction's city in the Vale of Eternal Blossoms:

Skydancer Shun is the flying trainer for the Horde and is located on the landing platform at the Shrine of Two Moons in the north.

Cloudrunner Leng is the flying trainer for the Alliance and is located at the Shrine of Seven Stars in the south-east.

This ability can only be learned at level 90, thus it is not possible for you to fly in Pandaria until you're level 90, unless you have a level 90 friend with a two+ seater flying mount.