Like it or not, you teach your children every day. Begin inculcating a sense of order, discipline and love of learning. Cultivate readiness - don't just wait for it. Don't try to ignore or work around your pre-schoolers, but include them in your plan. Gradually lengthen as attention span and ability increase.
Many states have compulsory attendance laws that begin at age six.
Children who are not yet able to read can still learn an enormous amount by hands-on projects and listening to good books. With an older child, you may begin home education at any time, but it is usually better to withdraw from a public or private school at the beginning of the school year, or at semester break.