Remove turkey tenderloin from refrigerator and let sit at room temperature for 15-20 minutes. Pat completely dry with paper towels, removing all surface moisture. Trim away excess fat or silver skin using a sharp knife and remove any remaining feather quills with tweezers if needed.
In a small mixing bowl, combine garlic powder, onion powder, smoked paprika, dried thyme, dried rosemary, black pepper, salt, and dried sage. Whisk until evenly distributed with no clumps. Set aside 1/2 teaspoon of the blend for finishing if desired.
Brush the turkey tenderloin all over with olive oil, ensuring complete coverage on all sides. Sprinkle the seasoning blend generously over the entire surface. Use your hands to rub the seasonings into the turkey, pressing firmly so they adhere well to the surface.
Set air fryer to 360°F (182°C) and preheat for 3-5 minutes. Lightly spray the air fryer basket with cooking spray to prevent sticking. Ensure the basket is completely clean and dry before adding the turkey.
Place seasoned turkey tenderloin in the air fryer basket, leaving space around it for air circulation. Cook at 360°F for 12 minutes without opening the basket. Carefully flip the tenderloin using tongs and continue cooking for another 10-13 minutes until internal temperature reaches 165°F.
Insert a meat thermometer into the thickest part of the tenderloin to ensure it reads exactly 165°F for food safety. If temperature is lower, continue cooking in 2-minute increments, checking frequently. The exterior should be golden brown with a slightly crispy texture.
Remove turkey tenderloin from air fryer and transfer to a cutting board. Tent loosely with aluminum foil and let rest for 5-7 minutes. Slice against the grain into 1/2-inch thick pieces for serving.