In Connecticut, about 24 percent of the approximately 39,000 kindergartners who start school each year are 4. But in the poorest districts, where parents may not be able to afford day care or preschool, 29 percent of kindergartners start at 4. In the wealthy ones, it is 18 percent. About 2 percent of kindergartners in those wealthy districts start at age 6, compared with fewer than 0.1 percent in the poor areas. The proposed change in Connecticut would take effect in 2015.

“It’s a glaring weakness that we should have fixed long ago,” said Mark McQuillan, Connecticut’s previous education commissioner. “Many of the wealthy parents enroll their children at 6 or 6 ½, and other families — particularly poor families — enroll their children as early as 4 ½ because they need the school support. It’s a huge developmental span.”

Some research suggests that children who enter kindergarten later perform better on standardized tests, but critics contend that family background and preschool experience often have a bigger influence on academic success than age. In any case, they say, such benefits disappear by middle school.

Indeed, Ms. Mead and others point to research linking a later start to higher dropout rates down the road, and to lower lifetime earnings because they begin their careers later. Some parents and teachers say redshirting — a term borrowed from college athletics, in which students are pulled from participation to prolong their eligibility — can exacerbate problems like bullying and low self-esteem among teenagers.

The Connecticut Education Department has not studied the effects of age differences on achievement, but some kindergarten teachers have reported that their youngest pupils are more likely to miss class, have difficulty focusing and generally require more handholding.

Jennifer Dominguez, a kindergarten teacher in Hartford, said she felt so strongly that 4-year-olds were at a disadvantage that she held back her own son, Kobe, until he was 5; he will turn 9 on Dec. 30. “The January birthdays are so much more mature and able to handle the curriculum,” she said. “The October, November and December birthdays, they’re just learning about what school is.”

Courtney Gates-Graceson, a lawyer in East Lyme, Conn., decided to enroll her son, Sebastian, who turns 5 on Sept. 29, in a $14,000-tuition preschool rather than to start him in kindergarten. “I don’t want his academic enthusiasm to be quashed if he can’t compete with the older kids in his class,” she said.