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Usually it's best to wait until they are at least 15 months of age before breeding. Even though the early maturing breeds do reach puberty by the time they are around 7 to 9 months of age, it is best to wait until they are around 13 to 15 months of age before you can breed them. This is because it allows them to grow more, increase their pelvic area and gain enough condition that can allow them to sustain themselves throughout gestation. Heifers that are bred too early tend to have too small a pelvic area to calve out, so some "whoopsie" heifers need to have a C-section done on them, or have the calf pulled. This can be quite costly, as the new calf will often have to be bottle-fed to get enough milk for him.
Occasionally, though, some heifers that are bred too early are able to pass and raise a calf without human interference, either at calving or during the period the calf is being raised.