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At 25 and 30 years of age humans fully develop their brain, this is known as the brain of a fully developed adult human, Itachi never had the opportunity for that..
Kids start losing their baby teeth when they're around 5 and keep losing them by the time they turn 13 (generally speaking). And Itachi had, by that time, seen a war, been sent to a dangerous organization which made him kill people older than him, saw his best friend die, had to carry out the massacre, including killing his parents.
All before he lost his baby teeth.