The best guess of neuroscientists and psychiatrists is that this happens because of a major transformation in our brains at that point in life.
[R]emarkable changes occur in the brain as it prunes away neuronal connections and makes the major transition from childhood to adulthood. Irwin Feinberg, professor emeritus of psychiatry and behavioral sciences and director of the UC Davis Sleep Laboratory [tells us], "Our outcome confirms that the brain goes through a remarkable amount of reorganization during puberty that is necessary for complex thinking." Feinberg explained that scientists have generally assumed that a vast number of synapses are needed early in life to recover from injury and adapt to changing environments. These multiple connections, however, impair the efficient problem solving and logical thinking required later in life.From here.
A study together with two related prior studies of the same children described in the linked story above tracked EEG readings of sleeping children over ten years (for two nights ever si months) has directly documented this transition taking place.
EEG fluctuations during the deepest (delta or slow wave) phase of sleep, when the brain is most recuperative, consistently declined for 9- to 18-year-olds. . . . This led the team to conclude that the streamlining of brain activity — or "neuronal pruning" — required for adult cognition occurs together with the timing of reproductive maturity. . . . [S]ynaptic density in the cerebral cortex reaches its peak at age 8 and then begins a slow decline. The recent findings . . . confirm that the period of greatest and most accelerated decline occurs between the ages of 12 and 16-1/2 years, at which point the drop markedly slows.
"Discovering that such extensive neuronal remodeling occurs within this 4-1/2 year timeframe during late adolescence and the early teen years confirms our view that the sleep EEG indexes a crucial aspect of the timing of brain development," said Feinberg.Psychosis, the leading theories to explain it hypothesize, happens when defectively thin connections between different parts of the brain lack enough capacity to distinguish between true sensory signals and noise, leading to cognitive hallucinations as the mind tries to make sense of mere random noise, or emotional whiplash as false triggers for mood changes cascade into intense mood swings that have no real environmental basis.
In childhood, the excess of synapes in place before adolescent pruning of synapses provide enough excess connective capacity to make adequate signal-noise distinctions in the brain. But, when this excess synaptic capacity is pruned in adolescence, the too thin connections that remain can no longer do their job and the descent into psychosis begins.
The finding also have relevance for educators and parents.
The synapses that survive the pruning the adolescence are the ones the brain sees being used at the time.
Some kinds of learning, perhaps foreign language learning, mathematics, musical ability and the task of being able to override "what you think you see" to draw what you actually see, all require considerable brain plasticisty that is at its peak from ages eight to twelve, and may be profoundly more difficult to master at age seventeen and beyond. Our peak learning capacity may be in the late elementary and middle school years, with our ability to learn many kinds of things greatly diminished by the time we are college aged.
Given that developmental reality, it may be foolish to focus immense economic resources and attention of the best higher educational system in the world, while leaving elementary and middle school education to happenstance with far less attention and far fewer resources.