I went to About.com to get some extra info. Experts warn that age should not be the only factor. You need to consider the overall child and how he'll compare with his or her peers. You might want to ask your preschool teacher for a kindergarten readiness screening. The teacher should also evaluate attention span, motor skills, socialization, overall behavior, independence, and interest in learning.
We decided to start my son in preschool this year, even though he's not quite three! We're starting with 2 mornings a week. If we hold him back for kindergarten, he'll be in preschool for 3 years. After reading all the details on the subject, we decided to wait and assess his academic and social readiness when the time comes.