The age at which women enter first marriage is known to be a major factor in marital instability. But to date possible differences by race/ ethnicity have not been examined. We use data from the 2006-2010 National Survey of Family Growth to examine differences by race/ethnicity in the shape of the curve relating women's age at entry into first marriage to marital instability. We find that for non-Hispanic white women, the probability of dissolution falls with age up to ages 30-32 and thereafter the curve flattens out. For black women, marital instability decreases with age only up to ages 24-26. For Hispanic women, marital instability falls from age 20 to 21-23 and then the curve flattens out; beyond ages 30-32 the curve turns upward. We suggest explanations for these patterns based in part on differentials in the associations of age at marriage with education and non-marital fertility. For white women, but not for their black and Hispanic counterparts, delayed entry into marriage is associated with a small increase in non-marital fertility and a pronounced increase in education. The common practice in the demographic literature in the U.S. of conducting pooled analyses - with simple controls for black, Hispanic, and other - can lead to misleading conclusions. Our findings underscore the desirability of conducting separate analyses by race / ethnicity wherever possible.