The answer to 'fence yellowing' is that the most common causes are sun exposure, moisture damage, and old age. Symptoms include the wood turning a dull, yellow or golden color, and the surface becoming rough and splintered. Solutions can range from simple cleaning and staining to complete fence replacement, depending on the severity.
For minor yellowing, homeowners can often DIY by power washing, sanding, and applying a new stain or sealant. However, if the damage is extensive or the fence is structurally compromised, it's best to call a professional landscaper or fencing contractor to assess the issue and recommend the best course of action.