I can definitely relate to this. I'm 27 and with the exception of a few things changing around me in my life, and the fact that I've gained knowledge that I didn't have when I was younger, I'm very much the same person I was a decade ago (or more). Part of this is fine, as I'm a firm believer in staying in touch with your inner kid; probably my favourite moments in my adult life so far have been when doing things that I could easily have done when I was eight. It's also problematic for me in many ways, though... and the problematic aspects very much tie in with some of my mental health issues, especially my depression, eating disorder and BPD. I function like a mentally-unstable teenager a lot of the time and I'm finding it very hard to move forward in life... guess that's why I've started therapy yet again. I know I have a great amount of anxiety surrounding "growing up", despite the fact that I don't even know what that really means a lot of the time, and I often feel very fearful of the world in general. I feel like I've spent my entire life trying to shield myself from getting hurt, because the few experiences I've had with that have deeply wounded me. Possibly it's because I'm such a sensitive and emotional person... though of course, not everyone who is sensitive and emotional develops mental health issues. I've spent a decent amount of time discussing the concept of being a "highly sensitive person" with my therapist though, and it seems like that can predispose one to various problems.
Aaaanyway... back to the child in an adult's body thing. I remember the first time I realised, probably sometime in my mid to late teens, that I'd never once imagined myself growing up. I know most young kids don't think about the fact that someday they'll get a job, own/rent a place to live, have romantic/sexual relationships, start a family, pay bills, etc... but there's more to it than that for me. Not only do most of those things seem entirely foreign to me (how I ended up living in my own apartment and then getting married, I really have no idea; they both seem like random flukes), I'm scared as hell of most of them -- and thus far, I've exempted myself from having to take part in the majority of them. I just never saw myself becoming an adult when I was younger.... I truly didn't. I thought it was something that "happened" to other people, much like "other people" get in horrific car accidents, get diagnosed with terminal illnesses, etc. Despite being 27, I truly don't really consider myself an adult in any other ways besides the fact that my chronological age tells me I am and that intellectually, I'm probably on par with many other adults.
It's funny really... being an only child growing up, I spent the vast majority of my time with my parents and people their age, doing and talking about the same things they were, and often didn't relate to other children very well. I was the youngest of all my cousins (the one closest to me in age was seven years older) and there were no other kids near where I lived. I was very much a "little adult" a lot of the time; my parents didn't do the best job at navigating the child/adult divide, and I always saw myself as more or less equal to them. I didn't take kindly to being treated as a kid and found it rather insulting.
Hrmm, maybe that doesn't seem so weird after all, given my upbringing... intellectually I've always been rather adult-like and emotionally I've always been very childlike. Makes for very confusing mental terrain, though. I get both "old soul" and "immature" from other people (the latter not really a compliment, haha), and I always feel so very at odds with myself.
