Back in the beginning stages of World of Warcraft, mounts were attainable at level 40, and were at least a good 60g + to purchase. Epic mounts were about 500g gold.
It wasn’t rare to see a level 40 without a mount, scrimping for another two or so WOW power leveling for a mount. I recall not even having an epic land mount till Burning Crusade. But now mounts are acquirable at level 20 (regular land mount), 40 (epic land mount), 60 (flying). But despite the lowered levels, mounts still require planning in advance. But on top of saving up the cash for them, mounts also come with a host of ways to gain them, to ride them, and to make riding even faster.
Ways to get them.
Between the main racial factions alone, mounts are readily available to purchase from your home faction. Doing reputation turn ins for other racial factions, or quests (such as for the Netherwing faction) will earn a player a mount at exalted.
PvP mounts are available through farming certain battle grounds for tokens and a bit of honor points.
Certain mobs (mostly) in instances drop rare mounts; these guys can be farm able but be prepared for hours and or weeks/months of grinding that same mob.
Certain professions such as Engineering and Tailoring can craft profession specific mounts as well.
Flying versus land.
Riding has various ride speeds. Land mounts start off at 60% speed increase and the epic ones a 100% increase. But flying mounts have the extra bonus of 170%, 270% (and even more if it’s a rare mount) and the whole flying thing.
Land mounts are good for the levels before level 60, but as soon as flying mounts are available, most use those as mounts as they can both fly and ride on land.