We often deride others by referring to them as childish. Why is this an insult?

We often deride others by referring to them as childish. Why is this an insult? What's so bad about being a child? The only major disadvantages of being a child I can think of are physiological and intellectual, and yet when we say someone is acting childish, we usually don't mean they can't perform complex reasoning or that they haven't reached the peak of their physical prowess. So how are we to understand accusations of childishness as insults?

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