It’s amazing how early the differences between boys and girls become obvious. The Little Guy is only 13 months and already he has developed a strong interest in ‘vrooming’ cars (or anything resembling a car) around the house. The girls never developed such interests. (Of course neither did Buddy, but that’s because for him everything is some kind of flying machine.) The girls have always shown more interest in play that relates some how to domestic things – dolls, cooking, etc. It is obvious though that they have an older brother since they are usually setting up house in a space ship or war zone of some kind. 
An incident that has really stood out to me in showing the difference between boys and girls occurred when Buddy was about a year and a half. Our neighbors had a little girl about his age who was very interested in dolls. One day her mother was showing Buddy how to hold one of the dolls. He gave it a brief once over before chucking it over his shoulder and heading for the toy cars.
It’s also been interesting to see how the ‘experts’ have changed their minds on the subject in the last few years. When Buddy was born it was still the prevailing thought that children were blank slates and now they are starting to admit that a great deal of the differences between boys and girls are actually hardwired from birth. Of course, anyone with half a mind would think this obvious.