Great points @eagle1 (along with everyone else)!
A lot has been covered (and probably more by the time I finish typing this), but I figured I throw an additional perspective into the mix.
In terms of my peers (mid/late-twenties to early thirties) I don't think being perceived as childish or immature is really much of a factor. As others have noted above, they're proud to enjoy Disney and other animated movies, YA fiction in print or on the screen, and a ton of other things primarily targeted at or otherwise intended for a younger audience.
Similarly, the quality or complexity of the art (or lack there of) may initially turn some people off, but I'd argue the writing can more than make up for that. Some of the most popular, long running webcomics or animated shows are quite simple (or even crude) and may superficially appear childish, but enjoy a massive adult following due to clever writing, mature references, or otherwise tackling complex themes that belie their true depth.
Gaming has certainly become increasingly mainstream lately, but is still somewhat more polarizing than the previous examples, and seems to depend more on whether or not they played games growing up.
Comics, on the other hand, definitely continue to carry a "nerdy" / "dorky" / "geeky" stigma (not as in smart or complex, but in the classic, unfairly stereotypical sense of awkward, introverted, alternative, obsessive, unpopular, exclusionary, or otherwise "different"), rather than something that is mainstream, popular, or "cool." Despite the continued, growing success of big-budget movies, games, and other media based on comics/graphic novels, along with the shift in attendance at Comic-Con and other events... I think the associations with Comic Book Guy and the "kids who chose the library over the playground at recess" persist, at least in the US (full disclosure: I spent plenty of afternoons reading, drawing, or playing Magic indoors, so I'm not ragging on anyone).
On the consumption side, many people have rightly pointed out that reading a comic (or reading anything longer than a tweet or FB post, really) takes more effort and attention than TV, streams, music or many mobile games. It also often requires more active, sustained attention, and is more difficult to multitask. I'm not sure I agree that comics are more difficult to consume than standard books... if done well I think they might actually be easier and convey the same (or greater) density of information with fewer words (because, hey, pictures). Truly appreciating them takes more practice than simply "reading," but you don't necessarily have to deeply understand paneling to enjoy the flow of a well made comic, just as you don't need to appreciate nuances in cinematography to enjoy the style of a well made movie. If you do, you may enjoy it on a different level... but it could be argued that the goal should be for you to enjoy the experience without noticing and focusing on why you're enjoying it. @eagle1 is right that the onus is largely on creators to determine how accessible their content is to readers... which I'd argue doesn't necessarily have to translate into watering things down, but rather distilling them. The hardest things to design are usually the things that end up appearing super simple.
On the creation side, however, there's definitely a certain barrier to entry, coupled with a lack of widely-recognized incentives. For example, practically anyone can press record on their iPhone, GoPro, or webcam and start creating video... and then get millions of views because they're cute, have an accent, or jumped off of something high and hurt themselves. Of course, consistently creating great video content, building an audience of loyal fans, and sustaining all that takes creativity and hard work, but getting started probably feels easier and more accessible than writing or drawing (let alone both). I'm not saying one is easier/harder or better/worse than the other, as it's certainly possible to put more or less time, effort, and skill in to any artistic medium... only that the ROI (at least the perceived ROI, if not real) seems to be lower. Hopefully we can help change that!
Whoa, what happened? I blacked out for a minute and suddenly had written way too much 