Get a good start with a Good preschool
Obviously all children are different. Some children are very social and learn well in an environment with other children. At the latest I would have them attend preschool starting at 4 years old. If there are any concerns with developmental with speak or fine motor skills the earlier the start the better. I would also check into Early Childhood Development Centers they are staffed with experts who can offer important interventions. The earlier the help the better if there are any concerns.