Because other activities that these ten and twenty-somethings engage in are so mature compared to gaming, *sigh*... and so is their behaviour.
It's a foolish statement intended to rub dirt in someone's face and make the person that says it feel good about himself/herself, feel superior and such.
...not that I can't understand where it's coming from with the medium being so damn immature and focused on "lulz MACHOOOOO!!!!!!!!!!!!! WARRRRRRRRRR!!!!!!!!!!!!! BOOBBBBBBBBBZZZZZZZZZZZZ!!! DRUUUUUGZZZZZZZZZZZZZZZ!!!". It is stupid coming from the person that says it, who can't see the other brave experiments in gaming and the great storytelling within (Persona 3, Demon's Souls, Zelda and such) as well as the healthy release of tension in it. However, it is NOT without basis. The medium is, for fact, mostly IMMATURE, with the story being fed into you forcefully, with shitty writing and dialogue, and the same cheap tricks that Hollywood "Blockbusters" use to entice you into a story, which work for the duration of the movie, but never cut deep into your psyche and make you think and feel.
You know, after typing all this, I'm really not surprised at people calling gaming immature. That's how the majority sees it, and I can understand that.
You don't hear the same calls about books, music, and films. And that is why because those mediums got over faux-maturity long long ago, with deep and relateable works in any and all of them. Gaming is only evolving into that right now, but it is the hardest medium among the four to get into... and the one that has to work hardest to portray a story.
