Answer:
That really depends on when she was bred. The average gestation period of a cow (or heifer) is around 285 days. Thus, if a heifer was bred when she was 15 months old, then she would likely give birth by the time she is 24 months of age. If she was bred when she was 18 or 20 months old, then she would calve out when she's 27 to 29 months of age.