Pretty well any age, really. Bobby calves (those that are a few days old) can be sold for veal (but this is primarily limited to dairy, not beef cattle). Baby beef calves are those that are from 1 to...
That is actually irrelevant. A calf is called a calf until it is around a year of age, until then it is called a heifer, bullock or steer. A heifer is called a cow until after she's either had her...