code: private LocalDate parentDateIncrement(final LocalDate date) {
          final int years = typicals.ageAtMarriage()
                  + typicals.gapBetweenChildren();
          return plusYearsAdjustToBegin(date, years);
      }
docs: Adjust the given date by age of parent at first child.
  
  @param date the input date
  @return the adjusted date