Blizzard has taken a major decision by changing the original game design direction to benefit fast power-leveling to 80, reducing the amount of time spent walking back and forth to accomplish quests.  You will be able to ride mounts at level 20, ride epic mounts at lvl 40 for merely 50 gold (training) and 10 gold (mount). Best of all, at level 60 in Hellfire Peninsula you can seriously reduce the time you complete quests by allowing you to fly at 150%.  At this rate, players might be able to reach level 80 in half the time it used to before, if not less.—source.

Blizzard Quote:
In the next major content patch, we intend to adjust mount requirements to further improve the leveling experience in World of Warcraft. In addition, cast times for summoning all mounts will be reduced to 1.5 seconds, down from 3 seconds. The following is a breakdown of the upcoming cost and level requirement changes we are making for each riding skill.

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

The game has gotten quite a bit longer, and allowing players to get from A to B faster only very minimally decreases difficulty; more so, it decreases time spent traveling. We don’t want to discourage players from trying out the game if they have not yet, and we especially don’t want to discourage existing players from trying out new characters because the leveling process from 1-80 seems too daunting after flying around Northrend grabbing purples for a while.

Whether you leveled your first character from 1-60 within a month of the game’s release when the leveling curve was less forgiving, and mounts seemed like a distant dream, or you leveled a character from 1-80 in the last month with mounts a little sooner, and experience gains a little accelerated from 1-70, no one’s game play experience and fun—the reason most people would play video games in their spare time in the first place—is at all cheapened.