The San Marino Unified School District will start a kindergarten prep class for children who are not believed to be developmentally ready for kindergarten. Kenneth Ogden, assistant superintendent of instructional services, said that while enrollment in kindergarten traditionally has depended on chronological age, developmental age is more appropriate. He said that although a child must be 5 years old before Dec. 2 to enroll in kindergarten, many children with September, October or November birthdays are not ready for kindergarten. After the two-year prep class at Valentine school, the children will enter first grade. Ogden expects 18 to 20 children to enroll in the prep class.