How old does a calf have to be before selling?

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 6 months of age. Beef steers/heifers/bulls can be slaughtered/sold between 6 and 18 months of age, if your purpose is for beef production.