こんにちは 皆さん！
So, I’m stuck on a (apparently) simple sentence:
I just wanted to tell my dad: “I’m an adult person! [and I can decide for my own life]”
My natural instinct was: “大人な人がよ!”
But then load of questions arose:
Thank you all!!
You can say だよ! You shouldn’t use です with your dad.
大人 is a noun, so it already means adult person on its own. You can’t use it as an adjective.
The particle wa is written は ‘ha’ in kana.
If I were to translate this, I, also female, would go for
わたし、大人なんだよ！(because) I’m an adult!
But of course shouting about being an adult makes you sound all the more like a child 
Shhh 
が is not an option for anyone to say in this context. There are many uses of が, but none of them would align with this.
Honestly, this depends on the Family dynamic. There are most certainly families where it would be proper. Here is a random question on the topic I found.
But I think more importantly “I can’t use だ cause I’m a woman” is a very incorrect statement and should be emphasized that that isn’t true.
父上、拙者は大人でございます。
Unless you turn it into an adjective by making it 大人っぽい, (but this doesn’t really help OP).
To say what you want to say （私は）自分で決められる would probably be just right. It comes off a bit brazen but it includes the implications of being reasonable enough to make ones own decisions.
Would just saying
もう大人でしょう！
also work in this case?
That is only if you are 中二病
A 中二病 would have used でござる
I remember 父上 because the American Indian-based character from Dragon Ball calls his dad that.
Why is it not true??
Why would it be true?
There are some situations where using だ sounds “rougher”, more “assertive” or “more masculine” but that is only a fraction of cases. But beyond that, だ is simply the copula, and would be used as such.
as far as i understand it: when we learn that an expression is more masculine or feminine it’s usually not directly about the gender, but about qualities associated with gender.
we can choose from situation to situation which manners of speech we use, but we also need to be aware of what they imply.
but that’s just my understanding, more experienced learners might have better answers.
I see, Tae Kim says だ is used mainly by men, but doesn’t go much into details… well, this confirms my initial thought: Tae Kim’s guide is not enough as a resource to learn Japanese grammar, it’s ok to have a preliminary overview, but then you need more detailed resources.
Thank you guys!
It depends on where it’s used. In many cases omitting だ is ungrammatical.
As for gendered language here’s a good article:
Using だ is just fine. 
One of the many reasons I don’t like Tae Kim is that he has no training or education in language learning pedagogy, and makes broad claims without explaining them and inserts his personal opinions without stating them as such.
Also, he has never been willing to accept that he’s wrong about anything. So, you get what you pay for.
@d-hermit I already don’t like that article since it starts with the feminine わ something that is near-dead in real life. I assume saying, “You sound feminine” is a nicer way of putting it than “You sound like an anime character.”
no, but you can say 大人の人 (it’s a の-adjective)