The mean age at first birth leveled off in 2004 to 25.2 years of age. According to our publication Births: Final Data for 2004 (see page 2):
The mean or average age at first birth for the United States in 2004 was 25.2 years, unchanged from 2003. Mean age at first birth for non-Hispanic white, non-Hispanic black, and Hispanic women was unchanged between 2003 and 2004. Mean age at first birth was highest for API women, 28.4 years, and lowest for American Indian or Alaska Native women, 21.8 years.
You can see the gradual upward trend since 1970 in this chart (click the thumbnail to enlarge):
You can view or download our report Mean Age of Mother, 1970-2000 or view/download a table containing the mean age at first birth from 1970 to present.