I read this here from a manga
and it uses the word joshi and I believe the character it refers to is 20 so had me wondering is it fine to use joshi for a 20 year old or is that odd or uncommon>
According to an NHK survey, “up to high-school age” is the most common usage, though “up to 20 years old” and “it doesn’t matter what age” also featured prominently. That said, the survey in question was conducted online, so it might not be a statistically random sample.
In the manga panel I shared does it mean the character is under 20?
What is it saying?
There’s no way to tell from one word usage. Things don’t work like that.
What it’s saying is that the speaker sees this person as a 女子, whatever age they happen to be. That’s all.
You’d be in the minority if you used it for a 20-year-old yourself, but it’s not unheard-of. People probably wouldn’t be giving you gasps of horror.
For context, what manga? What character? Given the use of じゃ, I get the feeling the character an old guy, so from his perspective, everyone would be a 若い女子. (Or possibly they’re from Hiroshima…)
Oh he’s an old guy yes.
Would most people use it for a 19 year old or is that minority as well?
People don’t think like that. Do you know the age of every person you make a comment about when you see them?
Just out of curiosity, you wouldn’t happen to the user who has been posting questions like this on Stack Exchange, would you? As was explained there, word usage in a manga can’t be used to determine character age.
19 is post-high-school, so that falls under the “up to 20” category. But don’t take speaking tips from old guy characters in manga.
Found another survey which actually spells out in detail the remaining responses to the survey, and in that one, 14.2% of respondents said you could always use 女子, so perhaps being 女子 is simply a state of mind. 
https://www.itmedia.co.jp/makoto/articles/0907/30/news023.html
So would using it for a 19 year old be in the minority? yes or no?
Yes
2. and what do you mean for the latter
3. True but at least for manga does the word mean readers are meant to see them as under 20
also sorry for the spam I tend to overthink
If an old guy calls someone a kid it doesn’t mean they are a literal child. This is like that.
You do know the age of every person you encounter? Or you never make a comment that could be construed as age-related unless you know their age? That seems difficult to avoid.
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What’s the tone or context for the panel posted
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What age range is joshi then?
You were already told in this thread that some people call everyone Joshi for any age so there’s no way to know.
No idea what the context is, you’ve only posted that panel.
It’s personal preference. It’s just that the largest percentage survey respondents lean towards “high school age”.
sorry to ask again so calling a 19 year old joshi would be in the minority then?
You’d have to ask the mangaka
I read these:
- The term 女子 in Japanese refers to a girl or young woman, but the specific age range it covers can vary depending on the context. Generally, 女子 is used to refer to females from early childhood up to their late twenties.
- As you can see, 女子 is the broadest of the three. Basically every female is included. And despite having the 子 kanji in there, it is not required that they be children. I’m not sure elderly women would be included, but the definition doesn’t exclude that possibility. In practice it tends to be college age women and younger.
Is this correct?
That’s an accurate summary in a sense, but literally only one person in the entire world can give you the answer you’re looking for.
We can tell you “yeah, about 20 is not unreasonable” but no one can make more fine-grained distinctions.
well would I be wrong to assume they are 20 for the usage of the word joshi?