Breed your heifer when she is 65% of mature size. More important than age to have enough growing done. A pampered heifer like yours is probably there by 15 months. Then allow for when you want the calf born. Not in the worst of summer or winter.
It is still best to be honest and truthful; to make the most of what we have; to be happy with the simple pleasures and to be cheerful and have courage when things go wrong.
Laura Ingalls Wilder