How big is it, and are its eyes open?

Size settles this faster than anything else. Compare against your own hand rather than guessing an age.

• Smaller than a tennis ball, eyes closed, no fur or thin furStill in or beside a nest.
• Fist-sized or larger, eyes open, ears standing upMoving on its own, sitting still when watched.
• Bleeding, limp, cold, or a cat or dog brought itAny of these overrides everything else.
• The nest was destroyed — mower, dog, or diggingYoung scattered, nest cup exposed.