Is it really? How intimate are you with Japanese culture? I am skeptical that Japanese people generally view anime with the same reverence that they view the works of Akira Kurosawa or Natsume Soseki.
edit: okay, anime is serious business.
Edited to add: both her and my parents are roughly the same age and share many interests in art/media/culture. Mine are from the US and were adults by the time Pong came out; despite the explosion of media in recent years I don't think the type of content they look for is much different than it would be otherwise. South Park might be the only exception.
The difference between intended audiences is more easily observed when you look at manga (comic books, often adapted to anime if successful) rather than just the animated cartoons, since the language used in manga directly reflects the target demographic. Plenty of manga are directed at an older audience and will use harder, more complicated kanji (Chinese characters), with or without furigana (helper text in a simpler alphabet) than those directed toward children.
I stayed a couple of nights in an inn as well and there were several shelves of manga (mostly hentai) on the bookshelf in the waiting room.
Also all of my Japanese friends in America (admittedly they were in their teens) were into manga and anime.
I think its a pretty well known and accepted fact that anime and manga are popular among all age groups in Japan.
They also seem to be fine (at least more fine than midwest America) with softcore porn. There was a lot of waste-up porn on news stands and I saw a row of Japanese men looking at more explicit porn mags in a bookstore in front of the magazine rack like it was a sunday paper -- totally nonchalant.
edit: fixed paragraph layout and spelling
Here is a list of the top grossing films in Japan[0] - note that #1, #5, #6 and #10 are anime. Windows in Japan has official anime mascots[1,2] and Mercedes has used anime to sell cars, at least twice[3,4]. Examples abound of anime covering adult (not necessarily pornographic but that too) themes[5], and of being used as a cultural referent to market to adults[6,7].
It's not even that controversial that anime is mainstream in Japan. They put Pikachu on some of their commercial airplanes[8], ffs.
[0]https://en.wikipedia.org/wiki/List_of_highest-grossing_films...
[1]http://www.digitaltrends.com/computing/microsoft-unveils-its...
[2]https://www.youtube.com/watch?v=fNfCEL4M_ac
[3]https://www.youtube.com/watch?v=qv7PodxyyWs
[4]https://www.youtube.com/watch?v=ucTwMQG1vuE
[5]https://www.reddit.com/r/anime/comments/239jkw/what_are_the_...
[6]http://en.rocketnews24.com/2015/10/27/rice-rice-baby-strains...
[7]http://en.rocketnews24.com/2015/10/10/japanese-employment-si...