Wood gets darker not only because of its natural process, but also because of air pollution, dirt, and foot traffic. The longer it's left untreated and unprotected, the more it will hold moisture, and that's what will cause serious damage. Because the gray represents dead wood cells, you may have to sand it down to get a good coatable surface. Deft Inc. Unlike what many people think, it's not the sun that causes the primary damage. Old Sol just starts the process by breaking down the wood so that it holds more moisture.
Moisture then provides the proper environment for wood-destroying fungus to thrive.
When you have wood that hasn't been treated in a long time, it develops cracks, and that's a fine place for dirt, soil, and mildew spores to collect. Therefore, when you apply a cleaning solution, be sure you leave it on long enough and be sure it penetrates these hiding places. Of all the ingredients a quality coating might contain, a water repellent is most important. You can clean a deck that's gray and has mildew, but if it has splits and cracks, it's much more difficult to protect. What will minimize that problem is a good repellent to stabilize the wood from moving through constant wet and dry cycles.