I don't want to suggest that kids aren't dumb, but there is more going on. At that age, they're actually morons because their brains just aren't developed enough.
Of course, if we weeded out some of the truly dumb ones...

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“Sometimes I think we're alone. Sometimes I think we're not. In either case, the thought is staggering.” - Buckminster Fuller

"A man talking sense to himself is no madder than a man talking nonsense not to himself." - Guildenstern, “Rosencrantz and Guildenstern are Dead”