In truth, your brain likely never stops 'developing'. We're learning things, updating our neural connections and networks, for as long as we live. If we didn't, we'd just be frozen in place. A non-developing brain is basically static. Which means, dead.
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But even if we did accept that the brain is 'done' at age 25, what we're actually saying is it's finished *maturing*. But that doesn't in any way mean that it's underpowered or non-functioning before then. Far from it.
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