In short, the analogy to athletics does not hold. The question we should ask instead is: What approach gives children the greatest opportunity to learn?

Parents who want to give their young children an academic advantage have a powerful tool: school itself. In a large-scale study at 26 Canadian elementary schools, first graders who were young for their year made considerably more progress in reading and math than kindergartners who were old for their year (but just two months younger). In another large study, the youngest fifth-graders scored a little lower than their classmates, but five points higher in verbal I.Q., on average, than fourth-graders of the same age. In other words, school makes children smarter.

The benefits of being younger are even greater for those who skip a grade, an option available to many high-achieving children. Compared with nonskippers of similar talent and motivation, these youngsters pursue advanced degrees and enter professional school more often. Acceleration is a powerful intervention, with effects on achievement that are twice as large as programs for the gifted. Grade-skippers even report more positive social and emotional feelings.

These differences may come from the increased challenges of a demanding environment. Learning is maximized not by getting all the answers right, but by making errors and correcting them quickly. In this respect, children benefit from being close to the limits of their ability. Too low an error rate becomes boring, while too high an error rate is unrewarding. A delay in school entry may therefore still be justified if children are very far behind their peers, leaving a gap too broad for school to allow effective learning.

Parents want to provide the best environment for their child, but delaying school is rarely the right approach. The first six years of life are a time of tremendous growth and change in the developing brain. Synapses, the connections between brain cells, are undergoing major reorganization. Indeed, a 4-year-old’s brain uses more energy than it ever will again. Brain development cannot be put on pause, so the critical question is how to provide the best possible context to support it.