My thirties were definitely a high point of my life. I felt balanced, peaceful, and physically and mentally strong, my career was at its highest point, my family and friendship circles were strong. I was studying calculus for fun, reading every book I'd always wanted to read, highly creative in my work, and physically active. I felt next-level zen. None of this description comes from a place of ego. At the time, I was just living and flowing. I did feel balanced, peaceful, and focused, but I wasn't aware of how special that time was until later. Looking back, I see it, though.
My INTJ SO had a similar experience in his thirties.
Forties? No. I think people in their forties are a mess. I was in a good place in my early forties, felt confident with who I was, and made huge progress with big life goals, but I crashed. It was the lowest point of my life for reasons I won't get into here. All around me, I watched my peers behave like a city on fire. Wreckless, irresponsible, immature, and mercurial, burning their old lives. It's easy to write it off as a midlife crisis, but the most interesting aspect of this stage is that it is involuntary, like puberty. Even now, watching younger friends, I'm like, "Mmm-hmmm." I see the immature and cocky ego, the internal fighting, and the poor decision-making. By the time you get out of your forties, you should be over all of that.
I'm now in my fifties. It's better than my forties. Life is much harder now, but there are aspects of my 50s that remind me of my thirties and the better parts of my forties. I'm convinced life keeps getting harder every decade, like the stereotypical video game analogy of "leveling up." I also have to say that who we choose to spend our lives with (friends, family, partner) deeply influences each decade as well as the mindset we CHOOSE to keep. Of course, not everyone has choices, but attitude influences our experiences.
As humans develop, our cognitive functions develop. By our late thirties/ early forties, our stacks should be fully developed. We learn a lot, we become well-rounded, and we have a chance to open up as people. Young INFJs can be a bit volatile and difficult, but as our functions strengthen, we grow into ourselves. So, you are right that 30s/40s is a special time for people.
I also saw my most shallow, sensor friends grow philosophical in their early to mid-forties, and start thinking about topics that were too deep for them earlier in life. That was...weird...but it is just the cognitive functions developing. It evens us all out a bit.
I'd have to look it up to see the exact age range, which I don't feel like doing, but our brains change around our forties as well. In our twenties and thirties, we are powerhouses. Later in life, we excel at a deeper kind of thinking that isn't as fiery. Our brains literally change from working like intellectual race cars to being more like deep pools of knowledge and wisdom. It is a physical change. No matter how successful people are early in life, the people who maintain happiness and believe they are successful at the end of their lives are typically those who stop fighting to stay fiery and competitive the old way, and find a new path, especially a path that hones what they do well and gives back to others. For example, many of the most successful people in different fields (historically in, for example, science or composing music) became teachers later on. We grow wise as we age, and it is better to use that wisdom by focusing on our crafts and giving and sharing. This is something INFJS are prone to do anyway. So, again, you are right that a person's forties are a special time because we have this opportunity to make a switch as our brains make a switch.
