Nothing happens at age 25

In the brain, that is…

Photo by Robina Weermeijer on Unsplash

I see lots of pop-science articles and Quora questions that refer matter-of-factly to a major transition in the brain at age 25. Some refer to the end of learning. Some refer to the peak of learning. Others seem to think we are immature until then and then we are suddenly mature afterwards.

Here are some of the headlines I got by searching for “age 25 brain”.

• 3 Things That Happen to the Human Brain at 25
• A Teen’s Brain Isn’t Fully Developed Until Age 25
• Why is 18 the age of adulthood if the brain can take 30 years to mature?
• What It Takes To Change Your Brain’s Patterns After Age 25
• Is 25 the new cut-off point for adulthood?
• Age 25 and the Male Brain.

Many of the Quora question demonstrate angst by the person writing the question, and that is why I’m writing this story of the brain. The questions come from people who see age 25 looming in their life’s windshield, and they come from people who see it receding in life’s rear-view mirror. The under-25 people ask questions about what they can do in the few years they have left to make the best of their lives. The over-25 people wonder if there is anything they can do to compensate for the poor attention they have paid to their brain health and training so far.

I hope that by describing what goes on in the brain up to and at that time I can help to clear up this misunderstanding.

The only thing that “happens” at age 25 in the brain is that synaptic pruning of the prefrontal cortex ends. Of course, “age 25” is really just a generalization; the final pruning can take place as early as 21. The “age 25” thing is just a safe estimate.

Synaptic pruning doesn’t affect the operation of the brain. The glia oversee pruning, and they are just removing synapses that aren’t used. Removing a synapse that never fires can’t affect how the brain operates, in either cognition or memory. So this is another meaning of “nothing happens”.

Synaptic pruning can sound like a bad thing, and there are certainly questions from people asking how they can avoid it, as if any hope of being a fully capable human being were being lost along with the synapses.

Synaptic pruning is a good thing.

When a baby is born, it has just about all the neurons it will ever have, 86 billion. Neurogenesis in a couple of places in the brain may add another million during life, but that’s a drop in the bucket.

The original 86 billion are connected by synapses in a rich, regular pattern as directed by our genetics, but our first knowledge is encoded in synaptic strengths, not the interconnections by themselves. Starting in the womb and accelerating after birth the strengths of these existing synaptic connections are adjusted, mostly higher, to represent early learning. This is why early learning is so easy for most of us: languages, songs, voices, faces, sports, stories such as history, and proficiencies such as math.

Since early learning consists of variations in a regular pattern, each generation, even each baby, learns what life is like in its own time, not in that of its parent’s or grandparent’s time, or of a distant ancestors’ times. This provides all of us the ability to thrive in each new generation, with its own new expectations and technologies.

Starting at about 7 years old, glial cells throughout the brain start recognizing that their own region of the brain has stopped rapidly developing, meaning its synapses are not changing strength much. This recognition starts at different ages in different parts of the brain, and continues for almost 20 years. As each area is recognized as not growing rapidly any more, the glia initiate the pruning process. Each area of the brain completes pruning a few years after it starts.

The pruning process involves removing synapses that are not active, through which no signals are being passed, represented by the strength of the synapse. Such synapses may have never been active, or they may have been active early on and then abandoned by further learning. However a synapse became unused, it becomes a candidate for pruning. Pruning involves destroying the presynaptic axon which is connected to the synapse. Fortunately, neuronal compartmentalization allows an axon to be destroyed without affecting the neuron it is part of or any of the other axons that are part of that neuron.

Pruning in each region takes years, during which time many of its axons are destroyed. If all the axons for a neuron are destroyed, the neuron itself will be destroyed since it serves no further purpose. If all the synapses of a dendrite are destroyed, the dendrite may be destroyed. Both of these outcomes appear to be rare, except in children who are severely neglected and thus have not been stimulated to use their brains sufficiently.

Now I want to talk about neuronal mass. Neurons have a cell body, called a soma, but very little of a typical neuron’s mass is located there. Most of the neuron’s mass is located in its neurites (axons and dendrites), which extend relatively long distances away from the soma. Most neurons have well over 90% of their mass in the neurites. The apparent purpose of synaptic pruning is to remove unnecessary neuronal mass from the brain.

The concept of unnecessary neuronal mass means that we are born with neuronal mass we will never use. Most of that mass consists of neurites that aren’t needed. While this may sound wasteful, it is the basis for both our early learning proficiency and our generational adaptability. This concept of building too much and destroying what is not needed is a common theme in biology. For example, our hands form in the womb as fins, then the fingers are carved out by apoptosis (cell death) of the parts between what then become the fingers.

There’s an interesting race occurring in the brain during later childhood into adulthood. Synaptic pruning starts at age 7 or so to reduce neuronal mass and continues to reduce neuronal mass until about age 25 in the prefrontal cortex. But we don’t often see neuronal mass dropping during that time. This is because a new form of learning is typically increasing neuronal mass at the same time.

Once a child has formed a basic understanding of the world, varying from 3- to 7-years of age, its learning changes from consuming its original fields of uncommitted neurons and synapses to using a new learning paradigm. In this new paradigm, new learning is stored as variations of previous learning. This means it reuses neurons holding previous information to store new information. It does this by extending new axons and new dendrites to other neurites in its vicinity.

This new, to each child, paradigm for learning is the basis of our associative memory and cognition, and it is the way we learn for the rest of our lives. With this new type of learning, we see every instance of the world as being “like” some previous thing we have seen. Every new thing is seens as a metaphor for something more basic, creating associations all the way down to our earliest memories.

This new learning paradigm involves building neuronal mass, and this is where the earlier mentioned “race” comes from. As synaptic pruning is reducing neuronal mass, learning in the new paradigm is increasing it. In effect, synaptic pruning is making room for the new learning to continue. Ideally, pruning makes enough space in the brain for a long life of learning within the associative paradigm.

The value of the associative paradigm then becomes apparent. Because it reuses existing neurons, it is much more space efficient than being limited to even a large array of initial neurons and synapses. More importantly, it ties together new and old learning that can reinforce each other. Last but not least, it means that every thought and idea is naturally associated with all the learning that preceded them.

A typical neuron at the time of birth has thousands of synapses created by our genes, and as I have explained, they don’t all survive. By age 25, that same neuron may connect to tens of thousands of synapses, almost all because of associative learning. By age 50, the same neuron may have have hundreds of thousands of synapses, representing the learning of a lifetime.

Let me describe how neurons are organized in a mature brain. By age 20 or so, almost all brain neurons are organized into small clusters of 10s, 100s, or occasionally 1000s of neurons. The neurons started out as a featureless array, but learning and pruning turned them into many clusters, each specializing in some small aspect of brain functionality. The neurons in these clusters primarily connect to other members of the same cluster, perhaps 90% to 99%. This means that most signals are fast because their connections are so short. Within a cluster there is very little myelination, so these are seen as grey matter.

Clusters are connected by relatively rare axons that extend out of the cluster to either neighboring or distant clusters. These longer neurons may be myelinated for speed, and if enough of them can be seen together, we see white matter. This organization of mostly local connections with occasional long connections is known as the small world network architecture. This mature organization is nothing like the original flat array at birth.

Now we can talk about age 25. Nothing is happening. Learning is taking place, but the learning is much the same as was taking place at age 16 or 24, or that will take place at age 26 or 46. The process of pruning is likely finished, though it may have been done at age 20 or 22. Most importantly, the brain is ready to spend the next 40 years or more learning new things. Some things will be easier or harder to learn than others, because of what the brain has learned up to that point.

What is critical to maturity is what is learned. The biology of the brain has little to do with it.