For the calf it is best to keep it apart from the older cattle in the first half year, with regard to transfer of diseases and viruses of older animals to younger animals. If this is not possible and you would likt to move the calves earlier, it is very important to realize what changes the calf has to face.

The moment calves will go to a yearling or cow shed they will be ventilated as cattle. This usually means a lower temperature and often ventilation with a lot of air circulation in combination with an outlet via the roof.

Is a calf able to withstand this and if so, at which moment and what are the risks?

Is is very crucial to know at what age a calf can keep itself warm, independent of ways of ventilation.

Around the age of 4.5 to 5 months the rumen of the calf is big enough (depending on growth and health).
This will enable the calf to take in sufficient kg / dm (dry matter) from roughage and concentrates for the animal to convert into growth and warmth.

If you move the calf too early – for examply directly after the milk period – to a space with a lot of air circulation without shelter for the calf, then the calf runs a greater risk of not being able to keep itself warm.

A calf of which the body temperature is too low will have problems with its organs. These will not function optimally, which results in malfunctioning of the rumen and intestines (e.g. a bad digestion of starter feeds). The calf's resistance will get worse, it will not take in sufficient energy from the feed and will keep a low temperature. All this will quickly create a bigger risk for a lung disease.
In addition, this kind of stress will provide all the space for coccidiosis to strike.