The average age to have first child

Going back to families and how here in the US the birth rate is declining and the average marriage age has sharply risen, I think one factor has been overlooked: what is the average age that americans are having their first child? According to the Alan Guttmacher Institute, in 2006 the average age for women was 25.9 and men was 28.1. There are several factors that can play into this high age: economy, rates of college graduates for men and women, and also the age of marriage. As for me and my girlfriend, we both are 20 and just ha our first child, Abel Lee. What other factors could contribute to this high average age? And what consequences could there be if that age changes?