First introduction to the concept I had is from mad magazine in a section of "Good side & bad Side of... jokes", a father looking at his teenage son reading a comic magazine and saying: "Comic Books, you're too old for this stuff", his son replies: "Not anymore, comics now have sex, nudity, violence...", the father pulled the issue from his son: "GIVE ME THAT, YOU'RE TO YOUNG FOR THIS STUFF".
I'm not saying there are no comics good for children, and I won't say there never was/is/will be children friendly comic books, all I'm saying is: "There are children comics, all ages comics, teenagers comics & adults comics"
'Marvel Adventures' series are kids comics, they still are good fun for older readers not too simple writings.
'DARK HORSE, DC & MARVEL' make comics for grown people that are still good for kids to read without any harm caused to the minds of the youth.
'Ultimate Spider-Man' is a good title made mainly to appeal to teenagers, it still is good for older readers. What is made for teenagers is something requiring of 'Parental Advisory' for those under the age of readers these comics are made for.
Spawn, Punisher & 'Mark Millar's' writings (except Marvel Knights Spider-Man) are directed for the adult audience, if comics were kids stuff then you should have no problem letting your kids read these comics, I point mainly on Spawn because I saw some of the animated series (have very mature contents, trust me).
Comic publishers rate the comics they publish for the age they fit, you see rating's like:
A = Adults, T = Teenagers, PG, etc......, the people judging comics to be kids stuff are most likely people who never read comics and heard they are kids stuff or they only read children comics.
Start reading comics or at least look at the rating label on the covers before judging comics aren't suitable for mature audience, start reading comics to see that reading comics is in fact a healthy hobby (don't focus on obssessed comic readers who want to be the characters in reality), comics fans desire art and enhance their drawing abilities and writing abilities.
Comics also are good for economy and lingual learning, a graduated student of English literature wrote his graduation thesis on Spider-Man and it wasn't tossed back to his face to write a thesis about something serious instead.
So as repetitve as I wrote it: "Comics isn't purely children items, comics reading is suitable for any age"