My guy is (per the dentist not the previous owner) in his 20s, and if I want to ride him without him dripping in sweat after 20 mins because it's in the upper 70s in the day, I clip him. Since his belly and chest are bare, he does get a light sheet if it's in the 40s and a fleece if it's 40 or below. If he wasn't clipped, I wouldn't even put a sheet on till it got down to about 40.
He recently had his teeth done and wasn't eating hay well, so that's when I first got his blankets. This was when Sandy came through and we had 20-30 mph cold winds at night and he got chilled. If it wasn't for the blanket he would've had no way to stay warm outside.