An informative article in Gamasutra about MMO demographics:

IGN Entertainment (...) has released a survey into the amount of time that devoted MMORPG players spend on their hobby (...) canvassed 5,634 of IGN's readers during the month of July across all of its member websites.

The survey found that the typical MMO player is a male between the ages of 13 and 34, with a mean age of 27. About half of them were heavy users, playing over 18 hours a week, and half more casual players who put in only 10-12 hours. The survey also confirmed that although many MMORPG players are willing to try new games, they tend to play only one at a time, indicating that the MMORPG market may not be expanding.

When choosing that single MMORPG, according to those surveyed, the most important feature was character customization. Problems severe enough to make players consider leaving are not enough new content, unbalanced classes, and too many bugs. Players generally were unwilling to forsake their favorite MMO even after reaching the game's level cap and exhausting its endgame content due to the community ties formed while playing.