Going against the trolls and people who obviously didn't play this I will step up and actually tell you what its like. This game has its moments of being fun, other things where it can be pointless and mind numbing to play.
But overall this game if left out of hand can be a drug. A drug that you cannot get off of without major efforts. However, when played sparingly this game is a decent way to pass the time when you have nothing important to do. So unless you have a great management of time, as in yourself, invent in those skills right away. If you are letting your child play this use the parental configures to make a set time your child is locked out of the game until the next day from the minute they are logged into a character.
The game is simplistic in its idea. You have two sides fighting against each other to control resources, territory, and ensure their survival in a constant world that goes into potential danger from the likes of the scourge, undead monsters, burning legion, demons, the blue dragonflight, magic using dragons set on ruining the balance of magic in the world, twilights hammer, a cult working with the black dragonflight, whos purpose is to cause destruction in the world.
When you first start the game everything is laid out in black and white what you are supposed to do. It is to start from level 1 and work your way, by questing, doing tasks for other people, grinding, killing for experience, dungeons, in which you work with four other people to clear, player versus player (PvP), teaming up with 9-39 other players to defeat the opposite faction on a pre-made area for earlier mentioned resources, and then eventually starting raids upon reaching the level 85-- in which you work with 9-24 other players to defeat various creatures that are determined to destroy the world, name Azeroth.
The violence in this game isn't horrible to say the least but sometimes can be a little iffy and potentially graphic, such as a cutscene of a mans face being melted off from chemicals dropped on top of him and many other people in the earlier expansion, wrath of the lich king.
In this game the language can be filtered by either completely removing the channel from the list of chats by a command of /leave [channel name here] e.g '/leave trade' and by setting the language filter. HOWEVER! This does not mean that you can completely shelter them from all the language as there are still places in which the language can still get to your child. The earlier cutscene mentioned has a few words go off in the form of 'd's but goes no farther than that.
There is some sexual suggestions but none that are not avoidable and sometimes occur without warning. There is nothing that can be done but to explain to your child if they are old enough. e.g 'the talk'.
There are some positive and negative messages in this game. Each message and potential moral that can be found in this game is open to being interpreted by the person and cannot be viewed the same way.
Overall, the game is unique all its own along with its ups and downs. Manage the time your child plays, if your child sees something in the game sit down and try to explain what is happening the best you can if it will affect their view on the world. The reason I say age 12 is because of the ability to understand and make their own decisions that can go out of your ability to control.