The body of a homunculus develops incomparably faster than a human's. Of course, mental and emotional development is an entirely different matter; after all, the pillars of a stable life can only be erected over time out of the joys and sorrows that pile up from moment to moment.
It's worth noting here that Nine was never given the chance to view from a different angle the values that were forced upon him, regardless of his desires or instincts. Those who forced their values on him had no interest in what a boy thinks or wants.