Now, I certainly think there is a place for more adult story-telling in the comic book medium; I'm not going to give an 8 year old Batman: The Killing Joke or the Watchmen to read. But I do feel that there is a distinctive difference between books suitable for all ages and those intended only for kids.
A good all ages story has elements that kids can enjoy, that adults can enjoy, and that everybody can enjoy. If you want to look outside of comics, Pixar's movies are a perfect example. The movies are marketed to kids, but adults enjoy them just as much or more.
Comic books, at least the mainstream Marvel and DC books, all used to be all ages! Stan Lee made a point of not writing down to his readers, instead he did his very best to challenge the reader to keep up! As a kid I always did well in subjects like language arts and spelling, and was told that I had a very large vocabulary. I attribute that directly to my love of comics, because I was exposed to these big words and concepts that prose books aimed at my age level could give me.
I look at my progression with Spider-Man as a good example of how stories can cater to all ages. As a kid, I was drawn to Spider-Man. He was brightly-coloured, funny, had awesome powers, and fought cool supervillains. In Grade 3 I tried my hand at my own Spider-Man comic, which was painfully brutal (but my Mom saved all the same, I wonder if she still has it...); the story featured Spider-Man foiling Electro from robbing a bank.
Here is the important part of that comic I tried to make; Peter Parker wasn't in it, it was all about Spider-Man.
As I grew older, I came to care about Peter Parker, the man behind the mask, more and more. I'm at the point now that I'm far more interested in Peter than Spidey; when I came out of Spider-Man 2 in the theatre, I commented that the movie was so good I didn't even care if he ever put on the Spider-Man costume. The life of Peter Parker was far more interesting.
Thus Spider-Man, for a very long time, was an all ages book. Kids might focus on the cool powers while adults (who still appreciate the powers and the fights, don't get me wrong) catch more of the nuances of the plot.
It takes real skill to do a good all ages book. The best example I can think of, in any medium, of all-ages writing is Calvin & Hobbes, by Bill Waterson. Kids love the strip, and adults love it just as much or more.
It's easy to throw in some T&A and make verything dark and gritty to make a book "adult" and "edgy". It's far harder to craft a story that everyone can enjoy, and it's a shame that more Marvel and DC titles aren't even making the attempt.
The other night my son was crying in the car, so after awhile I pulled out my tablet to read him a story. Unfortunately I discovered the storybooks I thought I'd downloaded needed an Internet connection, so I got the idea to read him a comic. I had downloaded some free comics from Marvel and DC's Comixology pages, and I figured I could just adapt the story to the pictures as we went; he's only 7 months old, he's not going to know the difference!
I chose "Billy Batson & The Power of Shazam", and let me say, I enjoyed the story more than my son did! Reading about a kid with the power of Superman, and how he uses it, was a wonderously fun adventure. The little touches like Billy transforming into Captain Marvel and then putting on civilian clothes to go to Parent Teacher interviews as his own father, that's exactly what a kid would do if they had the power to do so! I think the rest of the series is only $.99 an issue for the remaining 5 so I am going to be picking this up next time I am on Wifi.
All ages does not mean just for kids, there are lots of great stories out there without blood, guts, and women in thongs.