What demographic an anime belongs to used to be super important to me as a teenager. I got into anime thanks to shounen series like Naruto, but as I grew older, I wanted other people to take my hobbies seriously. I was determined to become a seinen purist. An effort that I would soon discover was utterly pointless.

For those who aren’t familiar with the terminology, anime is generally divided into 4 demographics: shounen, seinen, shoujo, and josei. Shounen and shoujo literally translate to boy and girl respectively, so in this context they refer to manga (and anime) that cater to young boys and girls. For shounen these are works like My Hero Academia and Fullmetal Alchemist, while shoujo can refer to anime like Yona of the Dawn or Cardcaptor Sakura.
Seinen and josei are used to refer to the grown-up alternatives. Seinen are manga for manly men and josei is for womenfolk. The exact differences that decide what mold any show fits in are loosely-defined at best and oftentimes lost in translation. For Western purposes, the main way in which we tell is whether a story “feels” mature. A metric that can’t be effectively measured.

Teenage Casper considered it very important that he watch anime like Berserk and Hellsing. Grown-up series that did away with the childish rubbish that prominently featured in “kiddie” shounen anime. What I really wanted was dark fantasy, edgy violence, and grim storylines. I was obsessed with Elfen Lied when it came out, but it’s not exactly a “mature” series. Elfen Lied certainly features sex and violence aplenty, just not in any way that reflects positively on the maturity of its audience.
That “seinen” doesn’t necessarily equate to “mature” is made even more obvious when you venture outside of action anime. K-On! is a seinen series, as are Hidamari Sketch and Lucky Star. Shows that teenage me wouldn’t have picked up under any circumstance.

In fact, the more you think about it, the more meaningless these terms become. Shounen may be directed at young boys, but can still be very meaningful to older readers/viewers. Seinen anime can be goofy and childish, as in Outlaw Star. Mahou shoujo stories, in spite of literally having “shoujo” in the genre’s name, are made for any demographic these days.
While trying to religiously stick to a demographic is ultimately a personal matter, these nebulous definitions can cause broader issues. Hanamaru Kindergarten was made somewhat frustrating by its author’s determination to make this a seinen manga. It’s literally a story about kindergarteners and their goofy adventures—perfect material for a kids show. Yet it excludes young viewers because it can never stop hammering on about the teacher’s fetish for large breasts. An abundance of fanservice doesn’t magically make a show more mature, but it’s the perfect way to get your work labeled as inappropriate for younglings.

Hanamaru Kindergarten discards a perfectly good concept for a kids show to instead cater to the questionably-lucrative audience of “seinen” fans that enjoy stories about toddlers. I sure hope those bouncing breasts and Evangelion references were worth it.
I had a similar experience when I was in my late teens. I wanted people to take me seriously even though I was bullied for liking anime. It’s one of the reasons why I got into seinen stuff like Monster, Yugo the Negotiator, Planetes, Gankutsuou, etc. I almost swore off most shonen except for Hikaru no Go and the original Hunter X Hunter (there was a time where liking HXH wasn’t cool). It still blows my mind how Outlaw Star was seinen the whole time.