Flying unlocked @ max lvl, with a lvl 90 req item from a vendor.
The faction that vendor belongs to, then offers you a "flying commendation" on exalted that unlocks (new xpac flying) for your entire account.
If you want to lvl before getting exalted on the main (lets say this is achievable within 1 reset after getting max lvl) then lvl with ground mounts, if you can wait 1 week, you instead can fly around while lvling.