The thermo-neutral zone is the range of temperatures between the lower and upper critical temperatures an animal can acclimatise to. In newborn calves, up to four weeks of age, this is between 10°C and 25°C. At one month old, it widens to between 0°C and 23°C.
Outside these temperatures — if it's too cold — calves must burn energy just to maintain their core body temperature.
Dairy breed calves don't carry enough body fat to maintain their own body temperature in cold weather, and feed is often limited by the number and size of feeds per day. Keeping a calf warm is therefore the most cost-effective way to make the most of the feed she's given.
The general rule for calf jackets, on young or sick calves: if you feel the need for a coat, so does the calf.