Heat pomegranate juice, sugar, and lemon juice in a saucepan over medium-high heat. Cook, stirring continuously, until sugar is dissolved, 3 to 5 minutes. Bring to a simmer; reduce heat to medium-low. Cook, stirring frequently, until reduced down to 1 1/2 cups, about 1 hour. Remove from heat. Let pomegranate molasses cool to room temperature, at least 20 minutes.
Preheat oven to 450 degrees F (230 degrees C).
Whisk the pomegranate molasses, horseradish, mustard, and sage together in a bowl to make the glaze.
Remove any giblets or gizzards from the turkey and place the turkey in a roasting pan. Brush with olive oil; season with salt and pepper.
Bake in the preheated oven, basting with the juices from the bottom of the pan every 15 minutes, for 2 hours. When an instant-read thermometer inserted into the thickest part of the thigh, near the bone, reads 175 degrees F (79 degrees C), brush turkey with 1 cup glaze. Continue roasting until no longer pink in the center, about 15 minutes more.
Remove turkey from the oven and brush with the remaining glaze. Cover with a loose tent of aluminum foil and allow to rest for 15 minutes before carving up that turkey.