A recent study conducted by Stanford University has established that parents who waited to enroll their kids in kindergarten by age 6 (and not 5) had better performing kids. The kids had better test scores and better self-control by the time they reached 7 and 11 years.
For a child to be ready for preschool , he, /she should be physically, emotionally, holistically ready to be in an educational environment that will have a group of other kids. Most preschools accept enrollment of kids at the age of two.