What is the youngest age a heifer can accidentally be bred?

This depends on the breed and environmental and nutritional factors. Early maturing breeds and/or herds that have highly fertile cattle can have heifers entering puberty at 4 to 8 months, which, if not weaned or separated from bull calves or the older bulls, will result in a "whoopsie heifer". The oldest a heifer be to reach puberty is 12 to 14 months.