When I was a kid I "seemed older". Now that I'm an adult, I keep getting mistaken for a minor

I'm not sure exactly why that is the case for you, but to me it seems that people are somewhat inclined to view teenage girls as "mature" because it's culturally normalized to sexualize girls from a young age, so they use their supposed "maturity" as justification. That also means that girls are pressured to look "like adults", especially in media. At the same time, people are obsessed with women's youth (sometimes teenagers are portrayed by 30 y.o. actors lol). So I'm not entirely surprised that people can't accurately tell the age of a woman who looks young.

I have experienced a similar thing. It was strange though because even though I 'seemed mature for my age' I was never taken seriously as a female human, even in literally almost identical/comparable scenarios. Sometimes older men/people can say this in order to groom you/ manipulat eyou into doing things they want. I thought this not takin gme seriously thing would change as I got older, and that once I hit certain 'milestone' ages and legit 'womanhood', I would finally be taken seriously as an adult human. I felt more embodied, more certain of who I was and more solid.

But really, I find almost the opposite is true. People, the patriarchy?, will perpetually try to infantilise women. And even if you don't subscribe and participate in this yourself through plastic surgery and youth obsession etc, they will still find a reason to treat you as LESS THAN.

Think about how our societies largely treats children? Generally not taken seriously as people in their own right. So it's been clear to me, this side of it, but the 'seem mature' side of it is less personally clear to me.

Objectively, I can think, oh yes its the over sexualisation of young women. But I don't subjectively get it.

I'm naturally soft and sweet but have had to learn to be much more 'severe', direct and 'masculine' in order for people to pay attention. Otherwise people just tend ot play out this low grade covert contemp. Ala misogyny.

The sexualization aspect has been covered already but another reason can be to justify parentification or neglect of children. It's mostly done by parents or caretakers who are not giving their children (often not all of them) age-appropriate support or care.

"She can stay at home alone just fine, she is so mature for her age"

"She doesn't need as much support/care/supervision/help. She's an "old soul""

"She can take care of the chores and all her younger siblings and be responsible for them. She is very mature for her age."

"My (minor) daughter is my best friend and she takes such good care of me. I can talk to her like an adult"

It's very common to treat the oldest sibling (especially the oldest daughter) like this.

Because of fashion and having kids later people can't tell 15 yo from 25 yo anymore. But you might also have a baby face, in that case, brace yourself, cause people will take you for youngin into your forties.

What is a minor, under 21/25? Women's ages are difficult to predict if you have a baby face. I'm ID'd all the time. It's a joke. I find it highly annoying and worse when told I should accept it as a compliment.

If a minor is under 18, I suspect you're in your early 20s and that's understandable. It can be hard to tell.