If you value your knees, you really should be covering your legs. I wear tights or knee coverings any time that temps are below 60 F. I would never consider riding in freezing temps without knee coverings.
That said, I notice a big difference between riding in the mid-20s and mid-teens. For one, my water bottle freezes solid during my one-hour morning commute when temps are in the mid-teens, but they just get a little slushy in the low 20s. My hands and feet are totally warm in the 20s, but they start to get chilled and a little numb in the mid-teens. In the 20s, I can get by with a single long-sleeve base layer under my jacket, but I will add a long-sleeve jersey for temps in the teens.