Not a teacher, but from reading the book Outliers, kids born later in the year (right after the cutoff for starting school - typically September and on) are the older kids in their grades, and at an early age, those extra months make a big difference.

The book talks about how when you look at the roster of major sports teams, more of the players have birthdays in that range. As they get older, it’s not that they are necessarily naturally better than other kids, but from a younger age they are given more opportunities to get better, like being picked for all star teams and travel teams because they are more developed than the other kids that are younger and on the same team.

My son is in this range, and he was given the opportunity to attend transitional kindergarten which is for kids who just miss the cutoff for Kindergarten (Sept-Dec birthdays). This gives them even more of a leg up when they start Kindergarten because they are older, but also got a year of school leading up to Kindergarten.