Why do people use the term 'Adult' to mean good?

[Adam Jensen]

C.S Lewis says it best:

Critics who treat adult as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence.

I used that against trolls arguing that MoP was childish and how they were somehow ashamed that MoP was for children (which it is not. MoP may look lighthearted, but I saw many undertones of darkness and conflict, but that's another argument.)

I think the inability to appreciate the things we appreciated as children, for fear of being seen as a child, is immature, just as C.S. Lewis says.

He also wrote to his goddaughter Lucy, as a preface to The Lion, the Witch and the Wardrobe:

I wrote this story for you but when I began it I had not realized girls grow quicker than books. As a result you are already far too old for fairy tales and by the time it is printed and bound, you will be older still. But some day you will be old enough to start reading fairy tales again. You can then take it down from some upper shelf, dust it, and tell me what you think of it.

Again, just to emphasis his point that appreciating things we deem as "childish" as adolescents/young adults is a sign of maturity as we grow older.

There's nothing mature about blood and guts and gratuitous sex. A mature story or game has difficult philosophical problems, shades of grey, light and dark moments. A mature story is mature because it'll take an adult mind to truly comprehend it, not because there are boobs and blood. And it doesn't even have to have these things to be enjoyable. Angry Birds doesn't and I can say I enjoy that game, at least on airplanes and car rides. And I most certainly don't have a problem with playing a panda in MoP, if only I can reactivate my interest in WoW.

"Rated M for Mature" is not a very accurate statement.

[Knight Gil]

There is a reason many people stop liking animation movies and other "childish" entertainment at a certain age (adolescence) and after that they feel much more free to watch whatever they want.

Simply put, teens want to impress their peers by doing more "adult-like" stuff. Some will stay on that phase for the rest of their lifes. Others will grow out of it and realize it's silly to try to impress anyone and it makes a lot more sense to watch and do whatever they like. Yay for LEGO!

[Reeve]

Some of my favorite movies are "kids movies." The new Pixar movie Brave, for example, which just came out, is probably the best I've seen this summer, including the Avengers. People who talk crap about movies or TV shows because they're "kid shows" just aren't secure enough in their own maturity to be able to recognize value when value is there. Doctor Who is one of the best shows on Television. The new BBC Sherlock series is too, and it's not full of sex and violence. That said, True Blood and Game of Thrones are also excellent shows. Don't cut yourself off from something because of someone else's preconceived notions.

[Minky]

I think you already know, and want to see a legion of others agreeing with you.

[checking facts]

because porn is adult and porn is good