Some things get better with age—wine, wisdom, and a worn-in pair of jeans, for example. Unfortunately, your joints are not one of them. In fact, more than 32.5 million American adults are living with osteoarthritis (OA), a.k.a. “wear-and-tear” arthritis. OA happens when cartilage—the elastic tissue that connects and protects the ends of bones in a joint—thins over time, says Kirk A. Campbell, M.D., orthopedic surgeon at NYU Langone Health in New York City. Cartilage can’t repair itself, and the damage can trigger inflammation, pain, and stiffness in the joint, he says. Here’s why it happens, who’s at risk, and more facts about OA everyone should know.