red cedar eventually goes from your nice light gray to almost black. I am not familar with white cedar.
Look around at other homes that are older than yours with the same siding and talk with them about sealants.
Cedar is a natural wood that is naturally immuned to rot and insects. Once you apply a sealant to the siding you have compromised its maintenance free aspect.
Here most people stain the shingles using a product from sherwin William's called woodscapes. Its a stain/paint that comes in all colors. This does protect the siding and will make it last much longer and will stop that darker gray and black color. The first application really soaks up, but after you seal the wood the next coatings will be much easier.