The correct term for a female cattle that has not yet had a calf or developed into the mature form of a cow is "heifer."
Heifers are typically around 1-2 years old and have not yet reached sexual maturity. Once a heifer has had a calf and reached sexual maturity, she is then referred to as a cow.
Stags are male cattle that have been castrated, while vealers are young cattle that are still nursing and have not been weaned. Yealing is not a correct term for a female cattle, and it is not commonly used in reference to any type of cattle.