Reddit mentions of Lifespan Development: Resources, Challenges & Risks

So this is outside of my wheelhouse as I'm not a developmental psychologist, but I've taken several graduate level development classes and I have a book in my office on lifespan development so I can try to synthesize an answer from it. Disclaimer, I am relying on the authors as experts and these many not be the only theories of development.

The book is "Lifespan Development: Resources, Challenges, and Risks" by Hendry and Kloep originally written in 2002.

So the question is basically asking if an individuals personality changes due to the changes an individual experiences during puberty. As a result, individuals who look like they have not hit puberty have not had these personality changes occur. The book has a short section on how puberty specifically affects development.

One of the major changes that comes with puberty is the changes we all experience to our body. So we get taller, our arms get longer, and we generally feel weird and awkward. All of these changes force adolescents to examine their own body as they're becoming more aware of their outwards appearance for several reasons. One of these reasons is this is when our reproductive organs are maturing and we develop much of our sexual identity. As we begin to notice the bodies of our peers maturing and feeling sexually attracted to them, we also self-reflect on what our own body looks like and how others may view it as being attractive or not. You could argue (although i'm not sure this is what the authors are getting at) that this self-reflection goes beyond just body image to how you're perceived by your peers. So changes in personality occur because we're more aware of how others perceive us than we are before puberty.

There are also all sorts of social pressures that come with puberty. As you begin wanting to date and have relationships with another individual you're changing and modifying your social structure as well. So you're reaching out to new individuals to join your social group (through a relationship) and you're doing more social activities as you go out on dates. In addition you're also supporting those around you who are attempting similar things as adolescents typically have to rely on each other for support in developing their sexual identity.

To answer your question, I would argue (based on reading this chapter) that the people who have not hit puberty yet may not have these shifts in their body and their social structure that force them to self-reflect and empathize with others, which is why they appear to not have gone through the same personality maturation process as their older looking peers.

EDIT: There are lots of other changes going on during adolescents as well, but I don't know how strongly they're tied to puberty. Example, there is other discussion in this thread about how the frontal lobes of the brain continue to develop and mature well into "adulthood" and these changes really begin in adolescents. I don't have a source to say these changes in the frontal lobes (and other changes) happen because of the same hormonal changes that cause our bodies to grow to look older as mentioned by the OP, so I have avoided discussing those changes in this post.