I think there is a big change at early 20s. The lack of parental guidance ends up being a big trigger I think to start developing your tertiary more. I've always noticed a huge difference between a recently graduated high school student to either a college student and/or someone working to make money for rent.
I don't think there is another big catalyst later on in life that everyone shares in order for them to fully mature. (Maybe children could be one?) The issue is I know a lot of adults who have hit retirement age and still haven't matured...
There's a few shifts. In the first years you develop your primary function (which I'm seeing in action with an Se-dom nephew at the moment, really interesting to observe). At some point (maybe from 4yo?) you start to develop your secondary and then slowly build mostly those over time.
A big shift comes in your 20s where you start really developing your tertiary, as stated above.
The one that is often forgotten though is between 40 and 50 years old, people really start developing their inferior function, even to the point where an ESTP could come off as an INFJ (for instance). This might actually be the biggest shift of all, since this is where we really get in touch with all facets of our personality and get full control over out brains. It's just not noticed as much, as we tend to be a bit more stable at those ages, meaning we don't often make a big deal out of it unless people start developing Se suddenly and get accused of having a mid-life crisis of course.
This early 20s trend is looking pretty consistent. It applies to me too.
From what I recall of being a teenager, realising one's personality was quite an introspective and selfish experience, though perfectly natural obviously, and any changes seemed to come about merely as a result of one's hormones and emotion suddenly veering off in a different direction. People around me, and myself for that matter, seemed to be putting a lot of effort into making their personalities known, but being so immature and self-involved doesn't really make for substantial realisations of one's character.
To put it simply, and without having any researched psychological understanding of the process, it seems that once you hit 20 you really have to start making yourself useful to the world, and to me that stands out as on of the first substantial markers for change in one's personality - it's less self-involved (though may be just as self-conscious) and it has to adapt what is around it, ie. other people and new responsibilities. Generally there is less supporting you also, and so your choices and decisions stick and define you far more as an independent person in comparison to teenhood, where a lot of the significant things that take place regarding your personality are the consequence of being hormonal and bat-shit crazy (I realise not everyone is like that in their teens, but take what I'm saying relative to your own emotional scale).
I'm really curious to know what the experience of coming into your own personality in your early twenties is like from the perspective of people who 'trickled' their way into those changes. I started university at 19 and my personality began pouring out of me and changing rapidly as their were so many changes in my life and so many more people to adapt to, but I have friends who never left home and just slowly 'trickled' their way into being independent, and I'm curious to know how their experience of coming into their personality contrasts with examples such as my own.
If anyone could enlighten me regarding that last point, I'd be interested to hear it
According to what I learned in University, personality is more malleable when one is younger and as one approaches the end of adolescence (between ages 20-25), personality becomes more static. Any major change in personality past that age would be the result of a traumatic brain injury or something to that effect.
Temperament, however, is generally the same from birth on. I don't mean temperament as in Keirsey's 4 temperaments, but more like someone who is "restless/anxious" or "calm" or "excitable". These tend to have more to do with biological wiring.
This applies to any model of personality which is based in trait theory.
Jung himself, however, seems to have conflicting views about his own theory (the precursor to MBTI) and he suggested that type could be changed at one point, but I haven't seen evidence that he had much of a consistent view of typology. It seems that his major part was identifying the dichotomies and explaining the cognitive functions a bit.
- - - - - - - - - - - - - - -
I have heard, though I haven't really tested it out, that it is much easier for someone to recognize an Introvert before age 20 than it is to recognize whether or not someone will be extroverted. I guess the introverts among us tend to be pretty obvious earlier on.
I've recently been reading some of Dario Nardi's work on Neuroscience (although his main work is still being shipped here). He's seen that we build up brain patterns where some areas of the brain tend to activate together (at the same time or in sequence). This leads to long-term connections being very obvious. Interestingly, with age people tend to get differing brain-patterns in their daily routine, while the long-term connections are still used on familiar tasks. I think he described an older INFJ that had brain patterns similar to ISTP's on short-term tasks, as he was training his brain to act in that way on other tasks.
I haven't had the opportunity to really delve into all of this, but this kind of thing would indicate that personality change would be mostly due to automating certain brain processes to make room for new patterns that could correllate to other cognitive functions.
I'm holding off on a verdict until I know more, but this would be very promising.
I would say it's more so event related than simply age related alone.
I have experienced a lot of things more comparable to 40+ year olds I know. I.e. Children, Marriage/Divorce, Education, Career, Loss of Parent, Gain & Loss of various forms of 'success'/materials
I don't simply think going through some motions which are related to adulting simply in themselves truly mark the sort of milestones in life one gains the most insight.
Having revelations & transitions is what makes people gain insight and effects the most change in personality.
But my honest ballpark guess if we are averaging age. It's 35-45 now a days. Why because it's the time period people are likely to have experienced a much larger chunk of life from the adult experience. Considering many are still on their parents insurance and phone plan until age 25 now a days I doubt most have simultaneously some how had miraculous discoveries of insight and it's the time of their most pinnacle growth of personality.