You really can’t set a specific age. It depends on the child’s development and social skills. It also depends on the child’s social needs. If she needs to be around a social environment with friends and things to do and is not as happy being home all day just with mommy, then it is something to consider as long as the child can communicate. A child that does not speak and communicate should not be in a school setting yet because it will only lead to chaos, IMHO.
Some schools want the child to be toilet trained before enrollment, others are willing to help you with the process. So that is NOT the biggest issue. The biggest issue is their ability to speak and socialize with other children.