The problem of income inequality often gets forgotten in conversations about biological clocks.
The dilemma that couples face as they consider having children at older ages is worth dwelling on, and I wouldn't take that away from Judith Shulevitz's essay in the New Republic, "How Older Parenthood Will Upend American Society," which has sparked commentary from Katie Roiphe, Hanna Rosin, Ross Douthat, and Parade, among many others.
The story is an old one—about the health risks of older parenting and the implications of falling fertility rates for an aging population—even though some of the facts are new. But two points need more attention. First, the overall consequences of the trend toward older parenting are on balance positive, both for women's equality and for children's health. And second, social-class inequality is a pressing—and growing—problem in children's health, and one that is too easily lost in the biological-clock debate.
First, we need to distinguish between the average age of birth parents on the one hand versus the number born at advanced parental ages on the other. As Shulevitz notes, the average age of a first-time mother in the U.S. is now 25. Health-wise, assuming she births the rest of her (small) brood before about age 35, that's perfect.
Consider two measures of child well-being according to their mothers' age at birth. First, infant mortality:
Source: Centers for Disease Control.
Health prospects for children improve as women (and their partners) increase their education and incomes, and improve their health behaviors, into their 30s. Beyond that, the health risks start accumulating, weighing against the socioeconomic factors, and the danger increases.
Second, here is the rate of cognitive disability among children according to the age of their mothers at birth, showing a very similar pattern:
Source: Calculations made for my working paper, available here. To match up children with their birth parents in the Census, I had to limit the sample to children living with two married parents, where both are in their first marriage, so it's a pretty select group.
Again, the lowest risks are to those born when their parents are in their early 30s, a pattern that holds when I control for education, income, race/ethnicity, gender, and child's age.
When mothers older than age 40 give birth, which accounted for 3 percent of births in 2011, the risks clearly are increased, and Shulevitz's story is highly relevant. But, at least in terms of mortality and cognitive disability, an average parental age in the late 20s and early 30s is not only not a problem, it's ideal.
But the second figure above hints at another problem—inequality in the health of parents and children. On that purple chart, a college graduate in her early 40s has the same risk as a non-graduate in her late 20s. And the social-class gap increases with age. Why is the rate of cognitive disabilities so much higher for the children of older mothers who did not finish college? It's not because of their biological clocks or genetic mutations, but because of the health of the women giving birth.