Perhaps, but as a childless adult who had to take over my parents' affairs as their physical and cognitive health declined, I marvel at the wonderful hits of dopamine parents get as they watch their children grow. It's an adorable perspective on life that I didn't get to share as my mother gradually forgot who she was.
This is a naive view of the world. Being childless is a qualitatively different experience for those in different walks of life. A childless financially unstable young adult will have a very different experience than that of a childless financially stable middle-aged adult.
edit: I am a man
You don't need to have kids to nut up and take responsibility for yourself and others.
Becoming an engaged father shifted my perspective on who I am, changed opinions on societal matters, and made me feel like the person I was -- despite, from a young age, spending non-trivial amounts of time on contemplating morality and society and considering myself as a youth to be "mature for my age" -- was a selfish git.
I went from "c'mon what's the harm"-ing naysayers to "HEY think big picture! LONG term!" on SO many aspects of life.
The man I was would not get along with the father I am.
Your statement won't be popular, but I agree that, statistically-speaking, it's an overt intellectual "next stage".
I'm not convinced it's automatically, or even usually, for the better. Many of the parents I know are deeply and profoundly unhappy.
As a childless person, I believe this is a societal problem, not a biological one. We've broken apart the tribe and made just two people (at most) responsible for most of child rearing. And worse, we pretend the parents are directly responsible for a child's safety and development at all times, even though we all know some kids are just way easier or harder to raise, right out of the box.
43-yr-old parent of 2. I love them. They're amazing. But there are so many challenging moments. So many.
In those deep/profound moments of stress, I try to remind myself that the only thing I really need to do is stay calm. Allowed to have emotions, course.
But to execute some level of calm really helps resolve so much of what you experience.
I mean, what is parenthood if not love and voluntary hardship?
On the other hand, I think you are describing your subjective experience. I've talked with some "one-and-done" parents who deeply love their child, but wouldn't want another one if you paid them.
"Happiness" is one of those words that has thousands of different definitions, so I would frame it this way: Parents almost always describe their children as the greatest joy in their lives, but (in America at least) they also generally express more dissatisfaction and frustration with their lives than childless people.
The authors charted human brain and divided it into "eras" where they saw significant changes based on age. Major life events can affect brain structure, and becoming a parent is one of the most important adult life events. Becoming a parent in early 30s is common. Just these facts combined mean that being in early 30s correlates with brain changes somehow. The authors explicitly mention that they know about this, and that they didn't control for this it yet.
Back to your question, I never said anything about maturing. It is a well-known fact, that female brain changes after childbirth. There is also research that suggests that first-time fathers brain changes too. This doesn't necessarily mean becoming more mature.
Retiring in late 60s. If you make it, becoming too infirm in body to get around in 80s.
These seem like brain changes at these transitions are more likely to be effects rather than causes.