I am uncertain about the meaning of ヒネた below.
子供の中にヒネた大人を捻じ込んだみてーな口調
I understand ヒネた子供 as a child who acts like an adult but I am not sure what ヒネた大人 means. I think it means an adult who got older, right?
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Sign up to join this communityIf it's a quote from a recent work, the author might be using ひねた interchangeably with ひねくれた. In that case, it's more about ill-naturedness than about age. This might be technically a misuse of the word, but a rather common one.
I'm not sure if we can say it's definitely a misuse, though, because there can be a continuity from aging to losing innocence to being cynical to having distorted (or ひねくれた) view of the world.
From a lexicographer's tweet:
https://twitter.com/IIMA_Hiroaki/status/211006532517773313
「◆ひねた子ども」と聞いて、どんな子を思い浮かべるでしょうか。ませた子どもか、ひねくれた子どもか。正解は前者です。でも、ツイッターでは〈ひねた性格の人〉〈ひねた大人に育ち〉〈ひねた目で見る〉などとあって、圧倒的に後者の意味で使っています。辞書に記述してもいいくらい。
The tweet was about the word's usage among Twitter users in 2012, but I think I see the same thing in a wider Japanese speaking population today.
EDIT: It might not be a recent change, actually. It's easy to find examples from 1970s.
This ヒネた is from type II or 下一段【しもいちだん】 verb 陳【ひ】ねる, which basically just means "to age, to put on years, to grow older". Used euphemistically and in the past tense / completed aspect (as when talking about a child's manner), it can mean something closer to "to seem older than one's years".
In your sample sentence, I think ヒネた大人 just means "an older adult".
Bilingual (albeit very minimal) entry:
If you can read Japanese, entries at Kotobank for: