Depending on the maturity of the bull, a twelve month old bull could be old enough--but I recommend 14 months in general. Also look at the size of the bull to ensure he's tall enough to service the angus cows and hereford heifers. Remember that murray grey originated as an angus/shorthorn cross, so you'll get less heterosis on the angus than hereford heifers. Easy calving, easy doing and generally excellent marbling, it should be a good cross. Daughters tend to be good milkers in general as well.