Both of my children have summer birthdays. Months before my oldest, Max, approached kindergarten, the questions from friends and family started. Everyone wondered if my husband and I were going to send him to kindergarten on schedule or hold him out until the following year.

The practice of delaying a child’s entry to kindergarten is called red-shirting, a term borrowed from collegiate sports where the youngest players sit out their freshman year to give them time to mature into better athletes. Malcolm Gladwell drew attention to the concept in his 2008 book “Outliers’’: “In competitive situations, a person who’s relatively older than the others will probably be the one who wins.’’

The practice of red-shirting has grown substantially over the years. The National Center for Education Statistics estimates that more than nine percent of children are age 6 when entering kindergarten — nearly triple the rate in the 1970s. In affluent communities, where parents can afford an extra year of preschool, the number can be even higher.

Parents seem to wrestle most with the red-shirting decision when they have a son whose fifth birthday falls just before the cutoff date for kindergarten eligibility, which, in Massachusetts is generally around Sept. 1. If the child starts kindergarten “on time,’’ he will be among the youngest in his grade. If he is red-shirted, he will be one of the oldest. More than 70 percent of red-shirted children were born during the summer months, and it is twice as common among boys as among girls, according to economist Diane Whitmore Schazenbach.

“Parents who have college degrees are twice as likely to red-shirt their children as high-school graduates are,’’ says Schazenbach, a professor of education and social policy at Northwestern University, who has conducted multiple studies on red-shirting.

“Generally, the research isn’t as clear cut as Gladwell suggests,’’ says Schazenbach, who used data from Project STAR, an experiment in which students were randomly assigned to classrooms prior to kindergarten entry. The random assignment of students to classrooms however, meant that pairs of children with the same birthday fell into different positions in their classroom distribution by the luck of the draw.

“The study shows that the benefit of being older at the start of kindergarten declines sharply as children move through the school grades. In the early grades, an older child will tend to perform better on standardized tests than his younger peers simply by virtue of being older,’’ says Schazenbach.

This makes sense: a red-shirted kindergartner has been alive up to 20 percent longer than his on-time counterpart, his brain has had that much more time to develop. The question becomes, does the benefit last? Schazenbach continues: “This initial advantage in academic achievement dissipates over time and appears to vanish by high school when the red-shirted student is at most 7 percent older than his peers.’’

The younger students on the other hand, experienced positive affects from being in a relatively more mature environment: in striving to catch up with their peers, they also tend to do well. “Because older classmates tend to be higher achieving and better behaved, they model positive behavior, and the younger students achieve great academic gains from learning and competing with older ones,’’ she says.

The decision to delay a child’s entry to kindergarten is typically made in the spring prior to the start of a school year, when information can be limited and uncertain.

“It’s a poor predictor of what their skills will be in September or October,’’ says Martin West, an associate professor at Harvard Graduate School of Education. “Their social and emotional development happens in spurts. A child can have dramatic progression in a matter of months, over the summer.

“In the region’s better-educated, more-affluent communities, there’s a perception that if parents don’t red-shirt a child with a summer birthday they will be placing him or her at a disadvantage,’’ says West. “But in our reading of the evidence it’s clear that they don’t need to feel that pressure.’’

Still, even with that knowledge, the pressure can be hard to resist.

Erin Mancinelli wasn’t aware of the concept of red-shirting as her son — who has an August birthday — approached kindergarten until the spring of his second year of preschool when fellow parents started asking whether she was going to hold him back.

“As soon as people put the idea in your ear, you can’t not think about it,’’ recalls Mancinelli. “I was thinking he would be fine to start and his teacher thought he was ready.’’

However, after a conversation with her father, Mancinelli’s opinion shifted. “My brother had a late August birthday and my dad reminded me that he always had trouble keeping up with his peers. My parents ended up having my brother do a post-graduate year before college so he could catch up,’’ she says. “Rather than having to do that it seemed better to delay his start now.’’

Yet for a long time, Mancinelli regretted her decision. Her son seemed bored in kindergarten, and didn’t feel challenged in first grade either. At the start of third grade, however, the family moved from Sandwich to Falmouth. “He had trouble adjusting and for the first time, the work wasn’t as easy for him,’’ she said. “I was really glad he wasn’t a year ahead at that point.’’

Schazenbach and West agree that the decision to red-shirt a child must be made on a case-by-case basis.

While the evidence of academic benefits to red-shirting may be unclear, some studies point to social boosts.

A study published in the National Bureau of Academic Research examined datacompiled among Danish students, where most children enter kindergarten at age 6. The study asserts that giving children a leg up in maturity and social/emotional skills results in significantly lower levels of inattention and hyperactivity.

The social and emotional aspect was what Norwell mother Megan Bloomfield was focused on when she decided to delay her son’s kindergarten enrollment for a year. “We didn’t do it for academics — because it seems pretty tough to tell if your kid is going to be an academic star in preschool — or for sports,’’ says Bloomfield. “He was very quiet and very shy. I wanted to him to have another year to develop socially.’’

Bloomfield’s son starts first grade this year and while Bloomfield says they struggled a little to keep him challenged in kindergarten, he was one of the most social kids in the class, a dramatic change from the prior year. “We didn’t hold him for what would happen in kindergarten or first grade,’’ says Bloomfield. “We did it for his whole school career.’’

I ended up sending Max to kindergarten on schedule. He has always been strong academically and he excelled in kindergarten and first grade. Socially, however, the situation is more complex. Max is more of a watcher than a leader, he’s not always sure of himself in groups. I’ve wondered if this would be different had he been old for his grade, rather than one of the youngest.

Still, I feel that we made the right decision with Max. So much so, that my daughter, Emma, who turned five a couple of weeks ago, begins kindergarten this year.

Jaci Conry can be reached at jaci@jaciconry.com