I'm still quite young, but I look even younger because I didn't actually go through puberty until I was 18 (it's a long story but the TL;DR is that I have a hormone disorder and I needed to go through hormone therapy so I wouldn't be a prepubescent boy for my whole life).
So even though mentally I'm an adult, physically I'm not really. It's getting a lot better, but from the age of around 15 or so until recently there was a huge disconnect between my mental age and my physical age.
All my friends looked and sounded like adults, while I still had peach fuzz and a voice that sounded like Minnie Mouse.
I often got mistaken for a girl (I have long hair too, which didn't help either) whenever I did anything even remotely grown-up, like going anywhere without my Daddy.
Or people would talk to me like I was a little kid, not knowing my true age. You know what I'm talking about, they'd talk in that obnoxious, condescending high pitched voice that you talk to dogs and cats with.
I still want to punch something when I hear people talk in that voice.
It was funny at first, but then it just got annoying, and I would resist leaving the house to do even the simplest of "adult" tasks, like going to the grocery store or the bank by myself.
Now that my voice has finally broken and the facial hair is coming in the disconnect is going away, so I feel more like a proper adult. Although I still kind of have a baby face (probably will for a long time), so my friends and I have a joke that before I was mistaken for a 12 year old and now I'll be mistaken for a 14 year old.