What is the source for this: "age increases the risk of poor outcomes from Covid-19. There is little argument that those over age 65 must take special precautions to avoid exposure to the virus."

The article you link to actually emphasizes that people of ALL ages are at risk. When you separate age from underlying medical conditions, age goes away. There ARE people over 65 with no underlying medical conditions whose immune systems are NOT compromised. Where's the evidence that they're at higher risk?