Older clear cases yellow because cheap TPU reacts to UV light, heat and skin oils over time. The good news: today's better clear cases use anti-yellowing formulas that resist this reaction and stay clear far longer.
To keep any clear case looking new: avoid leaving your phone in direct sun or a hot car, clean it monthly to remove oils, and wash your hands of lotions before handling. If a case has already yellowed, a gentle baking-soda clean can help — but a quality anti-yellow case is the real fix. When shopping, look specifically for "anti-yellowing" or "UV-resistant" in the product details for lasting clarity.