Regardless, a classroom with an 18-month age spread will create social disparities. “Someone has to be the youngest in class,” pointed out Susan Messina, a 46-year-old mother in Washington. “No matter how you slice it.” When Clare, her daughter, who is now 9, entered kindergarten at 4, Ms. Messina was aware of widespread redshirting.

“I thought, I’m not breaking the rules, I’m not pushing her ahead, we’re doing exactly what we’re supposed to do,” she said. “Then it dawned on me that in this day and age, there’s a move to keep your brilliant angel in preschool longer so they could be smarter and taller for the basketball team. But my daughter doesn’t need a leg up. She’s fine.”

Still, it bothers her that children in the same class are as much as a year and a half older than Clare. “She has friends who are 11 who are going to get their periods this year, and she’s still playing with American Girl dolls.” Another mother complained that her 4-year-old became hooked on Hannah Montana by her aspiring-tween classmates. A 6-year-old wielding a light saber can be awfully intimidating to a boy who still sleeps with his teddy.

At the other tip of the age span, parents who promote children to kindergarten before 5 are often seen as pushy, “even ogre-ish,” Ms. Messina said. But suppose your child is already reading at 4? Do you hold her back where she may be bored to tears in preschool or send her into a classroom of hulking 6-year-old boys? In 1970, 14.4 percent of kindergartners started at age 4. That figure has dropped to less than 10 percent.

The self-esteem movement has inspired parents to care as much about emotional well-being as academic achievement, and with fragile self-images still in the making, the worst fear for parents is setting up their children for failure. One Connecticut mother in Fairfield County sent her October-born son to kindergarten at 4, despite “the informal rule of thumb that everyone holds back their September to December boys.” Kindergarten seemed to go well, but when her son entered first grade, she said, “I got hit over the head. They told me he was way behind.”