I've had Jersey heifers get bred at 4 months, more than once.  Neighbor once had a beef heifer get bred at less than 3 months, she calved before her first birthday.   As our vet once said, where there's a will, there's a way.

Edit to add we aborted the Jerseys, caught it when we bangs vaccinated at 6 months.   Now I routinely give lut a week after the cow/calf are away from the bull.