We have to always consider the source of statistics. The ICRW has vested interest in making marriage below the age of 18 look bad from every angle, including health wise.
Clair Whelan – who certainly is no advocate of early marriage of girls, makes it clear that the teens and early twenties are when a woman is “most adept at conception and carrying a baby”:

“The biological reality that female fertility peaks in the teens and early 20s can be difficult for many American women to swallow, as they delay childbirth further every year, according to the National Center for Health Statistics. In the District, the average age of initial childbirth was 26.5 years in 2006, up 5.5 years since 1970, the highest jump in the country.

While we may not be mature enough to conceive at a young age, nor should we, that is still when the body is most adept at conception and carrying a baby,” says Claire Whelan, program director of the American Fertility Association. “Our biological clock has not kept pace with our ability to prolong our life spans.” Stillman agrees, pointing out that research about advanced maternal age and motherhood today is clear: The older you get, the more difficult it is to get pregnant and the higher the chance of miscarriage, pregnancy problems such as gestational diabetes and hypertension, and chromosomal abnormalities such as Down syndrome, among other concerns. A study published this month in Autism Research found that the risk of autism increases with a mother’s age: Women over 40 were 77 percent more likely than those under 25 to have a child with the condition. (There was also an elevated risk when the dad was over 40 and the mother was in her 20s.)”

Ovaries have not adjusted to many women’s decision to delay having children

While she makes the values based judgment of ICRW that women are not mature enough psychology yet, she does not try and change the reality that this is indeed the best time for women to conceive and carry children.