This varies by race and income- for example, since 1990 the rates for white parents doubled from 15 to 28%, whereas on the other end of the spectrum the rate went from 63 to 69% for children of black parents. Breaking down by economic, education and other levels follow more or less similar trends- big jumps at the low end of the range, small jumps on the upper end.

Edit- another interesting tidbit, divorce rates are highest among those over 55; presumably, parents whose children are now adults are moving on, whereas younger parents simply aren't getting married in the first place.