How long can you keep bread flour before using it?

We keep flour in storage for long periods in a dry sealed area. Bread flour will last at least three years if kept dry. We just store in the paper bags the flour comes in these bags have a plastic mid lining to prevent moisture penetrating the inner storage area. We make our own bread and have been told that if the flour is very old then adding more yeast to the bread mixture will help make the loaf as normal. Today (26.9.09) we made a loaf of bread from flour that was stored in June 2006 and it was perfect.