It is not advisable to burn 20-year-old treated wood, especially if it was treated with chemicals like chromated copper arsenate (CCA). Older treated wood might have more chemicals in it. Burning treated wood, especially CCA-treated wood, can let out harmful fumes like arsenic, which can hurt your health. It's safer to not burn any treated wood, no matter how old. Try other ways to get rid of it, like recycling, reusing, or asking local experts for help. If you're worried, talk to waste experts or environmental agencies about safe disposal ways.