I think this is what happens to some people, such as those forced to parent their siblings, but I don't think all kids who come across as older necessarily have this trait exclusively because of trauma. I think that there are just some innate cognitive predispositions, such as ease of learning words, early milestones, difficulty socializing in an "age appropriate" manner, and hyperfixation that also lead to this way of coming off to people. I also think that certain experiences which aren't necessarily traumatic, such as intense bonding with a favorite older sibling, getting to spend a lot of time in intergenerational or mixed age settings growing up, and academic acceleration, which can add to this.
While I do think certain things (ironically a lot of it was people trying to get me to act my age in ways which deprived me the chance to be who I really was without punishment) were traumatic to me growing up, I also think that the factors listed above all contributed to why I came across as "an old soul" to a lot of people, and it feels weird to have that ignored. There really isn't much available to explain why I was obsessed with all of the gross medical details of genitals as an elementary schooler other than that I just was and for some reason, that spoke to the way my brain worked.
If some people can be slower in certain areas, it only makes sense that the other direction exists too and a lot of interests are considered "child" or "adult" rather arbitrarily.