While I have been able to find online that yes, people do age at slightly different rates, I haven't really been able to find any information about extreme cases except early onset of puberty.
Personally I have always seemed behind my years, not only mentally but also physically. Like when I was 8 I looked and acted about 4 or 5. When I was 13 I looked like I was only about 8 and had about the same level of self-awareness of an 8-year-old. When I was 18 I couldn't get into a 15 movie, and again had only about the same level of self-awareness and intelligence of someone just becoming a teenager. I'm 26 now and only just starting to be recognized as an adult by some people, but am still widely treated as a kid or a teenager. Thing is, I feel mentally at the stage of a teenager, I still feel very immature and like I can't control a lot of things that an adult can. When people describe what it feels like to be a teenager I think to myself "I still feel that way" like with regards to emotions feeling very intense and being like a sponge absorbing facts and still being somewhat malleable still trying to find my identity.
I think the only thing that flies in the face of that is that I hit puberty before the rest of my classmates did, but I still didn't look like a kid who should even know what puberty is.
I have a good friend who looked like he was in his early 20s when he was just 13. I remember because the first time I saw him I thought "he's way too old to be in this class isn't he?" and only knew he was a student because he was in uniform and had all his work out in front of him. I thought he had been held back but he hadn't, he just grew quick. Funny thing is he always seemed mentally far beyond his years, like he acted way, way older than 13 and seemed far more intelligent and mature than any 13-year-old I'd ever met. Though now he's in his mid 20s he looks more the part, but he could easily pass for 30 or even 35. He always appeared to get along with people far older than himself and doesn't really make friends with people his own age well. Funny thing is his younger brother seems to be growing at the same rate, looking, acting and thinking like an adult long before adulthood.
Is it possible that some people grow at a decelerated rate and some grow at an accelerated rate outside of the usual variations that are expected, without it being an adverse condition, and for mental and physical growth to still appear to coincide with each other? Like, it's expected when someone grows physically fast or physically slow it shouldn't really be tied to their mental development, but in my case and my friend's case they clearly do seem to be tied together.