Should you abort a pregnant cow if she is less than 15 months old when she becomes pregnant?

This is a question that you need to ask your own veterinarian, because so much depends on the size and breed of the heifer as well as the size, age and breed of the bull. If the heifer is a large, healthy continental breed and the bull a small, easy-calving breed, her age should not be a problem and she will probably calve perfectly well. But this is one instance where a reply given over the internet is not specific enough for your own circumstances, so please consult your vet.