My husband and I have three children and I notice our second and third child is developing a lot faster than our first. I remember potty training our first daughter was a nightmare. Our second daughter potty trained a lot faster.
Our six year old is in grade one and she was having problems with her reading. She started doing Kumon and her reading has improved a lot. Our four year old daughter likes to do Kumon with her older sister, so my husband photocopies them for her and she does them too. Her reading and writing is beyond a junior kindergarten level. Our third daughter is 20 months old and she tries to keep up with her older sisters. She likes to write and colour and she has excellent pincer grasp. She climbs the stairs and says a lot of words earlier than her sisters did.
I think children with older siblings develop faster because they watch what they do and listen to what they say and want to be like them. The younger children love and respect their older siblings and want to play with them so they try to keep up.
I hope our third daughter potty trains easily :-)