A commonly used rule of thumb is to put jackets on dairy calves as another level of protection when the ground is frozen. They help insulate them from winter temperature swings at least until their rumens develop and generate heat from fermentation activities.
Source: The Bullvine, March 1, 2021. Link. Calf jackets are most useful for calves under 3 weeks of age and especially for any calves that are sick and struggling with illnesses. One of the keys to maximizing profit and keeping young calves healthy is creating an environment that keeps cold stress down, since it contributes to low performance and increased rates of morbidity and mortality in the first few weeks of life.