Generally, an employer has a duty to not hire employees who may harm or pose a danger to co-workers, customers and others. Because of the rise of negligent hiring lawsuits, employers must check the backgrounds of applicants. However, employers should not use information in a way that may lead to unfair labor practices, employment discrimination complaints or other prohibited practices.
Still, not all personal questions are prohibited. Some may have a relationship to the job you are seeking and it is best to be honest with yourself about how personal issues might impact your ability to successfully perform job requirements. For instance, having children may prohibit you from completing travel that is required for the position. If you accept the position and then are not able to fulfill the job requirements, you may be back looking for a new position sooner than you ever anticipated. A question about your age may really be directed towards finding out if you of legal age, that is, over 18 years of age.