He has looked that same age for six years now, though. Obviously, he's been growing up steadily since WoW launched six years ago, and they have only now changed the model to reflect those passing years. Obviously, it would be great if they updated every in-game model frequently to show age, but it's just not practicable.

He's six years older than he was when Vanilla started. From an RP perspective, you just have to deal with the fact that he's been growing steadily, and the graphics are only now catching up to how he's been looking recently.

The same argument applies to Desolace and similar places. It would be awesome if the trees had been growing in realtime since the druids began their work (I'm assuming post-AQ -- around the time of the Dark Portal reopening, maybe?), but that's just not something that the devs could/wanted to pull off.

Anduin was 11ish when the game started. In the novel leading up to the events, just a few months post-Lich King's death, he's 13 (and a half, according to him). As said above, they finally just updated the model, as the joke he's "been 11 forever" was due to a graphics limitation, not actual measured time passing.

Particularly since canonically, last I checked, certain spans of time were considered between each expac; a year between classic and BC, a year between BC and Wrath, even though more liek 2 years real time passed. I think the canon time of Wrath matches its real world time, but it hasn't really been said clearly, I believe, given they added in the Stormrage and Shattering events in a nebulous "a few months after the Lich King falls" timeline.