"I don't consider RS to be a kids game at all. We deal with quite adult themes in many of our quests and much of the game offers complex gameplay. Pushing those aspects of the game so that more people are aware of them might help address this incorrect perception that your buddies have, but then, if they were "kids" when they started playing it then that's their own personal experience." —Mod Mark, via Reddit
If you're a long-time Runescape player, this shouldn't be surprising at all. Jagex has made a big push to get away from the idea of Runescape being a kid's game over the last few years, with many more mature and complex quests and puzzles. This is especially evident in quests; While Guthix Sleeps and One Piercing Note manage to be anything but kid-friendly. Outside of the player base, however, Runescape still has a reputation for being "baby's first MMO". Is it another of the game's legacy issues, as Mod Mark suggests? Yes and no.
I'm not saying that Mark is wrong in saying that the game has grown up, only that some of it hasn't. Quests are probably the best example of the more mature side of the game, but that trend is relatively recent; While Guthix Sleeps was followed by Myths of the White Lands, a quest built around fairly simple sliding puzzles and a poop joke. A relatively small portion of the game's nearly 200 quests have the kind of depth that we've come to expect since then, most of which are aimed solely at the lower and higher ends of the community.
Quests are only a small part of the game, though, and a relatively unpopular one at that. Mark mentions that Runescape offers complex gameplay, and again he is partially correct. The game's mechanics have also slowly been getting more complex and streamlined—the Evolution of Combat is evidence enough of this—but most of the game is built around the simple point-and-click gameplay that we've come to love and/or loathe. Its simplicity is both a strength and a weakness in this case: at best it makes the game accessible; at its worst it can't compete with other games on the market today.
The third piece of this puzzle is the game's image. Runescape's graphics match its gameplay in simplicity. In itself, that isn't a bad thing; most fantasy games today are stylized to some degree, if not outright cartoony. I admit that many gamers today judge games based on their appearances, so it's easy for a cartoony game to be seen as a kid's game, but this is one area where Jagex is not helping themselves. Their recent ad campaign seems to have been aimed at a younger audience, as is their choice to make Yelps the game's unofficial mascot.
They've taken steps to change their game's image, and at the same time there are factors that preserve it. Many of the traits that made the game kid-friendly back at the peak of its popularity are still present in some form or another, others are not. It's not quite a kid's game, but also not an 'adult' game.
 — Reddit