Becoming a mother used to be seen as a unifying milestone for women in the United States. But a new analysis of four decades of births shows that the age that women become mothers varies significantly by geography and education. The result is that children are born into very different family lives, heading for diverging economic futures. First-time mothers are older in big cities and on the coasts, and younger in rural areas and in the Great Plains and the South. In New York and San Francisco, their average age is 31 and 32.