The youngest in the class: what a birthday actually does

Somewhere in your child's future classroom there is a boy who is nearly a year older than the girl sitting beside him, and neither of them did anything to arrange it. A date on a calendar did. This page is about what that date is doing, and about all the strange places its fingerprints turn up.

The hockey players born in January

In 1985 a Canadian psychologist called Roger Barnsley was at a junior hockey game reading the programme, and noticed something odd about the birthdays printed beside the players' names. Far too many of them were in January. He and his colleagues went and counted properly, across the Ontario Hockey League, the Western Hockey League and the NHL, and found a clean downward slope from January to December in every one. In the Ontario junior league, roughly five times as many players had been born in January as in November.¹

Canadian youth hockey used a January 1 cutoff. A boy born on January 2 was playing against boys up to eleven months younger than him, and at eight years old eleven months is an enormous amount of body. He got picked for the better team, which practised more, with better coaching, against better opponents. A year later he really was better, and the head start had become invisible.

Malcolm Gladwell put this at the front of Outliers in 2008 and made it famous. It is worth knowing that the effect did not go away once everybody knew about it. When two researchers repeated the count twenty five years later, 30 percent of NHL players were still born in the first quarter of the year, against 32 percent in Barnsley's original season.²

The ones who make it anyway are better

The hockey story usually stops there, and it has a second half. Being born in the wrong quarter makes selection much harder. The late-born players who are selected anyway turn out to be exceptional.

A 2017 study in PLOS ONE looked at NHL performance rather than NHL rosters, and found the advantage flips. Among elite players, those born in October to December scored around nine more points per season than their January to March teammates, and earned about 51 percent more.³ The authors read it as selection: if you were the small one and you still made it, you had to be genuinely better to get through, so the late-born players who survive the filter are a more talented group.

The filter is unfair. The children who get through it anyway are not damaged by it.

The same pattern turns up in classrooms

Kelly Bedard and Elizabeth Dhuey looked for the hockey pattern in school results across the OECD, using international maths and science tests. They found it. The youngest children in a year group scored 4 to 12 percentiles below the oldest in grade four, and 2 to 9 percentiles below by grade eight.⁴ In Canada and the United States they were also less likely to go on to university.

Read those two ranges again, because the second one is the more useful number. The gap in grade eight is roughly half the gap in grade four. It is real, it is measurable across whole countries, and it is closing while you look at it.

The finding is not really about the children

Todd Elder took American survey data and did something clever with it. He compared children born in the month just before their state's cutoff, who are the youngest in their class, with children born in the month just after, who are the oldest in the class below. Those two groups are essentially identical. They differ by a few weeks of birthday and a filing decision.

The detail that turns this from a statistic into something worth acting on is where the difference showed up. A child's position relative to the cutoff strongly predicted teachers' assessments of ADHD symptoms, and was only weakly related to parents' assessments of the same child on the same measures.⁵ The children were being compared to the room they happened to be standing in.

The same asymmetry runs through other decisions schools make. Relatively older children are nominated for gifted programmes more often. Relatively younger children are referred for special educational needs assessment more often, with each additional month of relative age reducing the likelihood of referral by a few percent.⁶ None of that is a conspiracy. Teachers are asked to judge children with no yardstick except the other children in the room.

So should you hold a child back a year?

Some parents do. The practice is called redshirting, borrowed from American college sport, where a player sits out a season to keep growing without spending a year of eligibility. Applied to school it means keeping a child home for a year even though they are old enough to enrol, so that they start as one of the oldest in the room instead of one of the youngest. It is much rarer than the coverage suggests. Between 2017 and 2025 about 5 percent of American kindergarteners were held back a year, peaking at 6.4 percent in autumn 2021 for reasons that need no explaining.⁷ An earlier study using two national datasets put it at 4 to 5.5 percent, against a widely repeated claim at the time that it was closer to 17 percent.⁸

It is not evenly spread. Redshirting is more common among boys, among white children, and in low-poverty and rural schools. Notably, there was no evidence that children with weaker skills at age four were the ones being held back.⁸ The decision tracks the family more than it tracks the child.

As for whether it works, the most recent large answer is discouraging for anyone hoping to buy an advantage. NWEA followed a kindergarten cohort of more than three million children through to third grade. By third grade, the children who had been held back were performing at the same level as the children who had started on time.⁷ Social and emotional maturity is a separate question, and a real one. The test-score advantage does not last.

The strongest argument against everything above

If starting older is better, the effect should show up in adult life, and the best attempt to check that found very little.

Sandra Black, Paul Devereux and Kjell Salvanes used the entire population of Norway, and were able to do something most studies cannot: separate the effect of starting school older from the effect of being older on the day of the test. They used IQ tests taken at military enrolment around age 18, outside school. The effect of starting school older was small and negative. The effect of being older at the test was much larger, which suggests a good deal of the school-based evidence is measuring the second thing while claiming the first. Starting older also meant lower earnings until around age 30, because a year later into school is a year later into work.⁹

They did find real benefits: boys who started older had better mental health at 18, and starting older reduced the likelihood of teenage pregnancy. And a large Florida study running the other way found that older school entry increased college attainment and reduced juvenile incarceration.¹⁰ The literature does not agree with itself, which is the honest summary. What survives across all of it is that the effects are modest, they fade, and buying a year of age costs a year of something else.

Everybody else draws the line somewhere else

The cutoff feels like a fact of nature until you look abroad.

New Zealand is the interesting one. Starting on your own birthday means a class is never sorted by age in the first place, and there is no cutoff to be on the wrong side of. It also means teachers absorb new five-year-olds all year long, which is why some schools have moved to grouped dates.

And then there is Japan, where some parents have decided not to leave it to chance. Analysing national birth records, Hitoshi Shigeoka found that more than 1,800 births a year are moved from the week before the April cutoff to the week after it.¹¹ Parents are giving up a year of childcare costs in exchange for their child being the oldest in the room rather than the youngest. Whatever you think of that, it is the clearest possible evidence that people believe this matters.

What I would take from all this

Find out where your child's birthday falls against your state's cutoff, because that is the part with a deadline attached. Our guide to cutoff dates by state has the table.

After that, hold the research loosely. The gap is real, it is small for any one child, it is largest in the year when it matters least, and it closes. The part worth carrying forward is smaller than an enrolment decision. A room full of five-year-olds contains almost a year of development, and the adults in that room are quietly comparing the children to each other. If yours is at the young end of it, say so at the first parent-teacher meeting of the year. It takes one sentence and it costs nothing.