The book "Nurture Shock" has a chapter on this and it's very interesting. Basically the research shows that IQ scores are not stable in children in preschool/early elementary. So a child who is tested at 5 could earn a gifted score, but 3 years later they may be tested again and not score gifted anymore. Or vice versa. It has to do with differences in development (with kids being faster or slower) and also environmental influences. Kids who are in a home with highly educated, involved parents will score higher because they have more exposure to games, puzzles, advanced vocabulary, books, etc. My DS was tested at age 3.5 because of developmental delays, and then again at age 6, and his scores were dramatically different. In between age 3.5 and 6 he "caught up". So I can understand the rationale to wait to test, however school districts should still be differentiating the work for the kids who are advanced so they are challenged to their ability level.
