The “lengthening of childhood” is having grim effects on American academic achievement.

That’s the message of a new paper by Harvard researchers, who warn that there is a downside to the increasingly common practice of waiting until children are 6 to enroll them in kindergarten.

Known as “redshirting,” after the practice of letting college football stars take a year off so that they can start playing for the varsity a year older, bigger, and stronger, the practice is widespread: A parent decides to hold a child back a year before beginning kindergarten, and suddenly kindergartners are taller and faster and first-graders are more literate. Manhattan private schools call the extra year “the gift of time.”

The practice has grown substantially: In 1968, 96% of 6-year-olds were enrolled in first grade or above. In 2005, the number had fallen to 84%, according to the paper by the Harvard researchers, part of a series issued by the National Bureau of Economic Research.

Not only are children being held back on an individual basis by ambitious parents hoping to give them a leg up; public schools from Maryland to Arkansas are following suit by legislating that children be older when they enter kindergarten.

The paper lays out a case that the long-term costs of holding children back outweigh the supposed benefits. The result, the paper says, is that children across the country are entering primary school at older and older ages — and opening themselves up to a likelier possibility of dropping out with less education.

Even private school children suffer from “redshirting,” the paper argues. Though they are at a very low risk of dropping out of high school, entering school a year late means losing one year of work experience and salary.

It also means one fewer year that the children, once they are adults in the workforce, will pay into America’s Social Security system.

The paper’s authors are Susan Dynarski, a professor of public policy at Harvard’s Kennedy School of Government, and a graduate student at the Kennedy School, David Deming. They said that delayed entry’s most affected victims are children in public schools.

These children will arrive in kindergarten with a greater chance of dropping out once the law will let them. Previous research has shown that their late entry into primary school means they are more likely to drop out. And when they do, it will be with a year less of education.

In New York City, children must be enrolled in public school kindergarten if they turn 5 years old by December 31. For the city’s private schools, the deadline is September 1. Yet private school administrators and admissions advisers said that Manhattan schools often will not accept a child younger than six. They said the schools are hoping to give students a boost in maturity before starting their formal education.

According to the paper, titled “The Lengthening of Childhood,” states have followed parents in pushing up the average age of kindergartners.

In the last 30 years, nearly half of all states have increased the age at which a child can legally enter kindergarten. Maryland and Arkansas have both moved their cutoff dates to September 1 and there is currently a bill in the New York State Assembly to do the same.

Ms. Dynarski said there are several reasons why public schools might be starting children in school at an older age.

One is that high-stakes testing and No Child Left Behind have put pressure on states to drive exam scores higher. When policy makers notice that redshirted children get higher test scores, they propose that their states move the cutoff dates to earlier in the year — usually to September 1.

“The older kids do better,” Ms. Dynarski said. “That doesn’t mean that the kids doing better are doing so because they entered later. That might be misleading to policy makers.”

Another reason is that kindergarten today is more difficult than it was 40 years ago. State curricula are asking more of children, and many schools and teachers find that 5-year-olds can’t meet the new challenges.

Private school educators agree, and in New York even a child who meets the early cutoff date may be turned away.

“If you have a summer birthday, private schools will frequently say you’re too young, you need to wait a year,” the president of the private school admissions firm Abacus Educational Consulting, Emily Glickman, said.

The headmaster at Grace Church School in Greenwich Village, George Davison, estimated that more than a third of his first-graders are 7 years old. He said the older ages are a deliberate choice, made to ensure that the children are “happier and healthier.”

“When you give kids a little bit more time to develop, they find the tasks associated with school developmentally more appropriate, and so they are more successful with them than they would otherwise be,” Mr. Davison said. “People in the world who feel good about themselves are more effective adults, and more effective adults have higher income.”

The director of the Upper East Side’s Epiphany Community Nursery School, Wendy Levey, said that every year she has a couple of students, usually boys, whose parents decide to delay kindergarten and keep their children in nursery school instead.

“Sometimes, developmentally, it makes sense for them to wait a year, particularly for boys,” she said. “You’re never doing a bad thing by waiting a year.”