According to the U.S. Department of Agriculture, any food stored at exactly 0°F is safe to eat indefinitely. The USDA says that keeping food frozen "prevents the growth of microorganisms that cause both food spoilage and foodborne illness." But take note, this rule applies for meat stored at exactly 0°F without fluctuation—it must be uniformly frozen at that temperature to keep harmful bacteria away. The process of thawing and refreezing can disrupt the bacteria and mold that naturally grow in aging food, allowing them to multiply when the temperature rises enough.
While properly cooking and then eating food that had stayed frozen at 0°F for years won't necessarily get you sick, you probably won't like how it tastes. So the USDA recommendstossing uncooked roasts, steaks and chops after a year in the freezer, and uncooked ground meat after just 4 months. And frozen cooked meat should go after 3 months.