Self-rising, cake, pastry, and all-purpose flours, called refined white flours, should not be used if they are gray or yellow in color, if you notice any moist lumps, or if it smells sour, and get rid of whole grain flours if they smell unpleasant or if they taste off. The bottom line is, you can use older flour if it doesn't look or smell strange, but it's probably best not to make a habit of it.