Don’t get too wrapped up in the branding. Calling something a “Senior” feed or a “Competition” feed is more for marketing than anything else. If it fits the horse’s needs from a caloric/nutritional standpoint, I wouldn’t write it off just based on the name.
I tried my OTTB on TC Senior/SG as a 5yo because it was the lowest NSC/highest calorie option available to me, not because he fit the arbitrary age category for a “senior” feed. He turned his nose up at it so we tried Ultium Competition. He’s getting 6lbs a day right now in light/moderate work (4x a week at most). That’s putting weight on him nicely coming out of winter.