This is something I've noticed with everyone that has more than one kid, the first is a bit immature for their age and the 2nd and 3rd develop at an accelerated

This is something I've noticed with everyone that has more than one kid, the first is a bit immature for their age and the 2nd and 3rd develop at an accelerated rate. Something about constantly seeing and interacting with another child just 1 or 2 years older than them drives them to advance and try to mimic their behaviour.