Our Gemara offers several approaches to this question:
R. Yitzhak relates that in Usha a rabbinic ordinance was established that a parent should be lenient with his son regarding his studies until he is 12 years old, at which point he should force him to attend school.
This stands in contrast with the statement that Rav made to Rav Shmuel bar Sheilat (who was a prominent elementary school teacher at that time) that he should not accept students younger than six, but he should begin teaching children from age six onward and “stuff them like an ox.”
The Gemara closes with the teaching of Rav Ketina, who says that a person who enrolls his son in school before he is six “chases after him but will never catch him,” i.e. that he will always need to keep after his child’s health because the studies will weaken him. Another approach is that this is a positive statement – he will be so advanced in his studies that his peers will never be able to catch up. The Gemara also considers the possibility that this decision should depend on the health of the child.
Contemporary psychologists offer different opinions on the question of the proper age to begin intensive schooling for children. In any event, it appears clear that this decision should be based on the physical abilities of the child, and even more on the child’s emotional and spiritual preparedness for such study. Thus there is no “correct” answer to our question, as it depends on the development of each individual child. Nevertheless, the age range mentioned by the Sages in the Gemara is largely what is practiced even today, as those are the ages when most children are ready for formal schooling.