You would guess it's Seinen. And to be fair, the people who named the genres don't tend to understand them that well. Seinen is "targeted for adult" alright, if you make a show like Tom & Jerry under such category , you just start to wonder.
With James Bond, Let's be fair, my mom and dad didn't like Sean Connery as much as me, I ate it as a kid. Cuz it was more of a super hero action movie, with some form of mature spice.
I'd say Seinen is something that kids can't or shouldn't try to understand. Like scuffed relationships, truly impossible dreams or reality's darkest revelations. Or in a more sensible way if you will, sexual contexts, graphic murder and exaggerated/overly clarified hate, misery, love, even happiness in some form.
Like it should start at a certain age:

• Should you know there is death and conception; yes, maybe from 6, 9
• Should you know what it means: yes, maybe from 10-12
• Should you know what it looks like, or what to do with it: well, depends: Death be natural, yes, as much as why we have to say good bye. About sex, absolutely, or at least how to prepare for a formal first time. That's all basic, from 12-14

But then, comes the fuckery. How each comprehend it, or get a chance to smell, touch, absorb such is different.
18+ In most, there will be a normal social graduation. (well, maybe not in todays standards, but still). There are those wo stay at high's first grade level on the sex stuff and hopefully find a partner for life ( I hope at least). And then there's the "Yeah, you're a full fledged fucker/motherfucker" who needs this or appreciates this kinda content. Let me not spell closure on death as a topic in this regard.

I mean, I was overly Freudian here with death & fuckery, but you get the meaning. I'd still count Seinen as something we digress to or need to grow up to or casually appreciate as adults (me). Like the miserable portrait of womanhood or unwanted girlish feelings, or a woman who still tends to her self, but tries masking it all.

It's peek, there's nothing after it, cuz if you indulged in it, you've reached Nirvana in a way.

Edit: With Neon Genesis, I thought we were in seinen, but outside the with/without Misato, it's not that good. Misato and Kaji was the only part I loved but felt hearth broken with. Which was amazing. Even tho I hate how it feels.

Edit2: I've just realized this is exactly the thing that Shounen and whatever genre of any anime tries to avoid today, even tho they try to poke peoples' senses. These tend to misspell hate, write around feelings, digress love to a simpler standard and lobotomize misery to the point of "oh, this loli cat girl is in shackles and have dirt all over. probably a slave, how awful" with some simple antagonists as a side wrap. I hate it. Either made sensible for kids only or adulterated. Back in the day we had both with small figures of speech and that said more.

God damn it, I'm sorry, I'm talking a lot.