There’ve been adult animation productions for some time now and then, even the Flintstones aired at prime time as well as room for underground, even pornographic animations. It may come from Japan, but it can also occur outside of it as well as stuff that may be ambiguously aimed at children like Watership Down. Keep in mind that the context for a child-friendly film has changed.
There’ve been adult fans too but so far in my generation there’re more vocal ones out there especially online where many of them would’ve been exposed to Newgrounds cartoons at some point or another. They’re vocal enough to be catered down, spoilt if you will. When it comes to a vocal online fanbase, there’s got to be a reason why late night cartoon programming hit big time.
It may not always be causation or correlation but it’s coincidental enough to be linked on some level. Adult fans are more likely to be devoted to their favourite programmes than children do, a good case of brand loyalty. Surely it’s not mainstream but substantial enough to have a successful market and presence.