Josei = manga targeted at adult women
Shoujo = manga targeted at young women
Shounen = manga targeted at young men
Seinen = manga targeted at adult men

Looking at the words, we see some similarities:

Jo[sei] and [Sei]nen are manga targeted at adults
[Shou]jo and [Shou]nen are manga targeted at young people
[Jo]sei and Shou[jo] are manga targeted at women
Shou[nen] and Sei[nen] are manga targeted at men

Now, in the case of Shoujo, Shounen, and Seinen, the genre names follow the pattern of Age + Gender, but following that pattern, Josei should instead be Seijo. Does anyone know why Josei alone is different?

女性 - josei - woman
青年 - seinen - young man
少女 - shoujo - girl
少年 - shounen - boy

The genres are called what they are not because of a pattern.

They are the names of the seperate age groups/genders.

I might add, in case it were not immediately obvious, although josei and seinen share homophonic syllables, doesn't mean that those syllables share the same kanji.

It is in fact different, and only the sei in seinen could be construed to refer to age. (Which I assume you were trying to point out in your examples.)

Although, the homophonic syllables shared by josei and shoujo are in fact the same kanji, as with shounen and seinen, and shoujo and shounen.

In Conclusion (If it is tl;dr just read this part.)

Your comparisons are...

Jo[sei] and [Sei]nen are manga targeted at adults - Incorrect as far as the shared sei does not refer to age and is different kanji.

[Shou]jo and [Shou]nen are manga targeted at young people - Correct, refers to age (sort of, refers to amount, as in little. Shounen translated directly [Chinese style] is literally "little years". It carries a male connotation in Chinese as well though, and is used in the same context. Same with shoujo. "little girl")

[Jo]sei and Shou[jo] are manga targeted at women - Correct, refers to gender

Shou[nen] and Sei[nen] are manga targeted at men - Correct, refers to gender...sort of...(nen actually means year, in both Japanese and Chinese so I just accept it as being part of the whole word. )

Sorry for the language rant, just happens to be something I'm interested in.

Thanks for the explanation. I guess if the "sei" actually has a different meaning in Josei vs Seinen (and actually, my computer's font doesn't display Kanji, so I really appreciate you mentioning this), then that's probably the reason for the different order. But, after reading the other meanings (girl/years/little), I am really curious about the meanings of the two "sei"s.

Also, if shounen means "little years" and shoujo means "little girl", as opposed to "little boy" vs. "little girl" or "little years boy" vs. "little years girl" etc., does that mean the word "shounen" is older than the word "shoujo"?

Thanks again for the info.

I don't know if the shounen is older than shoujo.

the 'sei' in josei means something along the lines of personality, gender, disposition. So josei literally means something along the lines of female gender, or female disposition.

the 'sei' in seinen, means fresh/young in that context. It also means blue in Japanese, although that is a different pronunciation.

Read further for some additional information. To any native Japanese speakers, please feel free to correct me as all the Japanese I know is self taught.

Kanji have two pronunciations: kunyomi and onyomi. Onyomi is closer to the Chinese pronunciation and is mostly used in nouns or words that are made up of more than one kanji. Kunyomi on the other hand is the Japanese way of pronunciation, and is mostly used in stuff like adjectives and verbs.
Be aware that the above is a huge generalization and should not be used as a reference.

So the 'sei' I was referring to was Onyomi (being part of a kanji-fusion-word [that isn't actually a word, I made it up.]). When used in a context where it would be pronounced using onyomi, it usually means fresh/young (which was mentioned before.)

The kunyomi pronunciation is 'ao' and can have an 'i' added onto it (called an okurigana, think affixes,) to designate it as an adjective, or it can be used as 'ao' in a kanji-fusion-word (I facepalm everytime I see myself write this,) so case in point, the rule I stated above is an extreme generalization and therefore should only be used for ease of memory (forget what I said if you plan to learn Japanese.) The kanji 'ao' connotates blue/green/pale.

Onyomi and kunyomi are not the only pronunciations that exist for kanji, but they are the most common and you probably won't need to know any other.

Hmm, that's interesting... even though seinen is targeted at an older age group than shounen, the "sei" means fresh/young.... This is especially surprising if seinen is the oldest age category.

Hmm... in your earlier post, it kind of sounded like the genre names were names of groups of people before they were also used to describe manga genres, in which case, are there other such names of groups of people that aren't also genre names (e.g. middle-aged, seniors, etc.)? In that case, the fresh/young would make more sense.

I'm not sure.

I don't think there's a specific name for middle aged men, as far as I've heard, colloquially they're referred to as otoko, which is means man/guy.

But let's make something clear, 'shou' as in shounen and 'sei' in seinen isn't the same as English units of measure.

'shou' means a little. Of anything.
'sei' in that context is reservedly fresh/young in terms of age.
I would hardly call seinen the oldest category either. I believe it's mostly used for men in their twenties, however, dansei (I've actually never seen dansei used in my life, but then I don't read much Japanese besides visual novels and light novels. Novels are too hard for me, and manga is pretty much all scanlated, except for exceptions like Mushishi and others,) would be the true opposite of josei, and you don't see young girls referred to as josei that much, I believe that josei is for more middle aged women.

For reference:

Onna - Otoko
Josei - Dansei
? - Seinen
Shoujo - Shounen

Feminine and masculine, respectively. The English equivalents would be.

Girl - Guy
Woman - Man
? - Young man (twenties or something)
Little girl - Little boy