And actually I don't disagree. The manga is a seinen, definitely, I have only read a few pages but it had so many sexual references that can't be anything else but targeting a more "mature" readership.
Now, my point is, does it mean the anime is seinen too? Personally I don't think so.
Then I started to think about what the differences between both genre are. It should be easy, shonen is targeted toward a young male demographic and seinen toward an older one. Categorizing a manga is easy, you just look at the magazine it's published in, anime are not published so how to say what category(ies) an anime belongs to? At best you look at the hour it's broadcast, at worse, you wonder. (Or you don't care, that works really well most of the time)
There are a few things where seinen and shonen differ to me.
- The plot.
The more complex it is, the higher chance you have it'll be a seinen.
Which doesn't mean a shonen can't have a complex plot, but it's usually not the aspect you feel is concentrated the most on, while in the meantime a seinen plot is the main focus.
- The pace
Shonen is usually fast paced with a lot of action, seinen is calmer and takes its time. Just see the difference between things like Letter Bee which had fights all the time (which is already quite close to seinen in my opinion) and Mushishi.
- The themes.
What is the main difference between Gintama and Sayonara Zetsubou Sensei? (Which are both gags before being respectively shonen and seinen)
In the first Gin wants to keep on living and make the world a better place despite everything, in the second Nozomu wants to kill himself using the slightest excuse.
That's the main difference, it doesn't show only in the themes but also in how they're played, shonen usually has a lighter tone, seinen is more serious in dealing with the exact same themes.
- The atmosphere.
That kinda goes with what I was just saying, in how seinen is usually more serious than shonen where it's common to find some kind of humor, but it affects the art as well. See the atmosphere between a page of Bleach and a page of Hellsing for example.
- The characters.
Their age is one of the main characterstic, if they're around 15 it's shonen, if they're older it's seinen. There's also a difference in how they're developed. I won't say this applies to everything because it depends a lot but I do feel like there's more depth to characters in seinen. (That's why, if asked, I'll say Tiger&Bunny is a seinen, despite the appearances)
In the end I'm going to say something completely not helpful but, differentiating shonen from seinen is not easy, because what makes an anime (or a manga) belonging to a category or not is not a single thing, it's a mix of several.
To take the example that brought this whole article on, to me, while the manga is definitely targeted at 20-something years old people, the anime version of Brave 10 is a shonen, it has the action, the barely developed characters, the very simple plot and a pretty cheerful atmosphere despite everything.
Now take Guilty Crown as another example, I would have said it's a shonen before, but is it really? (Of course not it's Noitamina, young boys won't watch TV at this hour).
Telling seinen and shonen apart is definitely not easy, most of the time both genre overlap. I was saying Gintama was a shonen before but Gintama has different layers, it can be enjoyed by young boys but has so many things a young boys won't understand yet.
It all depends on how you read it.
(Also, I've submitted this to Nopy's Guide to Anime)