Sarah's* 14-year-old son loves football, and sport is a big part of their regional Victorian community, so it's a big deal that she's pulling him out of the game next year.

She just can't stand the worry anymore.

She says there's a "dangerous" difference between Joel* and his under-15s teammates: their size.

"He's just physically small for his age, so he probably looks like a small 12-year-old. He still fits into size 10 clothing ... and he's quite slender," she says.

Many of his teammates, however, are already "very well into puberty".

"Some of those kids would be shaving. They are fully formed men, some of them," Sarah says.

She's so uncomfortable watching Joel that she prefers not to.

"It's very hard to watch him run out on the field knowing that he's so small and physically nowhere near as developed as the boys he's playing against," she says.

"Any boy out on the field can get injured, it doesn't matter how big they are.

"But because he's so small, I struggle to watch him."

With such developmental variations, do we need to rethink how we're grouping kids in sports? Is it unfair to simply group them by age?

Maturing early a 'distinct advantage'

Jamie Salter and his team are working on an innovative solution to the problem of kids of different developmental levels competing together.

Mr Salter, a performance pathways manager at Swimming Australia, has been working with University of Sydney researchers on the nationwide project H2gr0w.

Their aim is to understand why girls and boys, at a certain age, have been dropping out of swimming.

It might come down to something as simple as when they were born.

There are big drops in swimming participation rates among kids aged around 13 and 18 years old. ( Getty: Hero Images )

"When swimmers start to go through puberty — so you're talking at ages between 11 and around 14 or 15 — the differences, and the speed and magnitude in which children transition ... can differ remarkably between boys and girls, and athletes of the same year," he tells RN's Sporty.

"If you're born early in the calendar year, or you are what was classed as an early maturer, you have a distinct advantage because you are physically stronger, maybe physically bigger, so you have longer levers.

"Therefore that gives you an advantage in the competitive element of the sport."

Mr Salter says kids who are younger than others in their age category, or less physically developed, are less likely to beat their peers.

As a result they're more likely to get discouraged, and drop out altogether.

Joel isn't keen to drop out of footy — he wants to be where his friends are — but his level of development is having an impact.

Sarah says in games her son is cautious, and holds back.

"You put him out on a field with boys ... who are huge — they hit pretty hard," she says.

The need to level the playing field

The challenge of grouping athletes of different physical development is one Swimming Australia is trying to tackle by making some changes — and Mr Salter says some other sports could stand to gain by doing the same.

"The philosophy and the process we've gone through could be mapped to other sports," he says.

Mr Salter says other sports might also benefit from adjusting how they group young athletes. ( Getty: AleksanderNakic )

As part of the project, Mr Salter and his team attended every state or regional swimming championship in the country, and watched 20 years' worth of swimming — half a million swims all up.

They also profiled 2,000 young Australian swimmers from 10 to 15 years of age, and tracked 300 swimmers, one on one.

Data from the research informs an algorithm that allows them to perform "corrective adjustment" on athletes.

Mr Salter says it's "basically like a golf handicap", designed to level the playing field within age groupings.

"We've been able to pinpoint how a difference in age and a difference in biological maturity can be equated to actual swimming time," he says.

Jamie Salter hopes changes at Swimming Australia will reduce athlete drop-out rates. ( Supplied )

That time is applied to younger born, later developed athletes, who can be "fast forwarded" and their projected ability determined.

"So we can run competition results and we can say to a swimmer, actually if you keep going at this, you'll be just as fast as the athlete that one won the race last week," he says.

"And that's kind of creating a different picture."

Mr Salter says H2gr0w is also about avoiding misidentifying athletes who, when placed with peers of like ages, might seem like they underperform.

"When we put the [corrective] adjustments in, they could be some of the highest potential athletes," he says.

'It's about athlete retention'

Mr Salter is hopeful the ongoing research will have a significant impact on swimming and swimmers both present and future, and help coaches find a solution to the mystery of exactly what stage of development each swimmer is at.

"Once that can be found and we can provide that, and support that for everyone in our sport, then the rewards are enormous," he says.

He also hopes to see a reduction in drop-outs.

"It's about athlete retention. We see big exit drops from our sport around 13, 14 [years of age] and 18, 19. It's to alleviate that."

Corrective adjustment wouldn't alleviate Joel's problems on the footy field, but other solutions might.

While Mr Salter says organising juniors to compete in categories according to size and maturation rather than age is "a little bit of a can of worms", some sports bodies are opting for it.

He says US Soccer is one of them, and the Australian Rugby League "have brought some weight categories in".

Joel's football coach encouraged him to join a team in a younger age category, but Sarah says he "didn't even entertain the idea".

He didn't want to stand out any further.

Sarah says it will be "a shame" when her son leaves football, and holds out hope that he'll "eventually catch up, physically".

"But at the moment he's not there," she says.

*Names have been changed for privacy reasons.