E.g. your lungs are still developing/growing, until your early twenties. But last I checked, everyone under 20 is still capable of breathing just fine. Better than most people, if anything
Because 'not fully mature' is in no way the same thing as 'doesn't work properly'.
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There seems to be this idea that an 'underdeveloped' brain is like a half-built house: not fit for purpose until completed
In reality, it's more like evolution. Every step on the evolutionary ladder is a fully functioning species, they just change over time
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