In the next major content patch, Blizzard will be making changes to mount requirements to help improve your leveling experience in WoW.

You are now able to get a mount at level 20 and your epic mount at level 40. The Apprentice Riding skill will cost you 4 gold and 1 gold for the mount. You can now get your flying mount at level 60. Also, it will only take 1.5 seconds to summon your mount, down from 3 seconds. With this new patch we will see the play time for everyone reduce a great amount.

Here are the full changes in the patch:

• Apprentice Riding (Skill 75)
•   60% land mount speed
•   Requires level 20
•   Cost: 4 gold
•   Mount cost: 1 gold
•   Mail will be sent to players at level 20 guiding them to the riding trainer
• 
  Journeyman Riding (Skill 150)
•   100% land mount speed
•   Requires level 40
•   Cost: 50 gold
•   Mount cost: 10 gold
•   Mail will be sent to players at level 40 guiding them back to the riding trainer
• 
  Expert Riding (Skill 225)
•   150% flying mount speed; 60% land mount speed
•   Requires level 60
•   Cost: 600 gold (faction discounts now apply)
•   Mount Cost: 50 gold
•   Can now be learned in Honor Hold (Alliance) or Thrallmar (Horde)
• 
  Artisan Riding (Skill 300)
•   280% flying mount speed; 100% land mount speed
•   Requires level 70
•   Cost: 5,000 gold (faction discounts now apply)
•   Mount Cost: 100 gold

Source: worldofwarcraft.com