Their visual judgement and motor skills have not fully developed before then - putting them at risk each time they negotiate a busy street, according to new research.
A study using a simulated virtual traffic environment showed accident rates can reach as high as eight percent among six year-olds.
Even those aged 12 were struck by vehicles two percent of the time - and had to compensate for their lack of judgement by choosing bigger gaps in traffic.
It was not until early adolescence youngsters got across without incident, according to the findings published in Experimental Psychology: Human Perception and Performance,
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Psychologist Professor Jodie Plumert, of Iowa University, said: "Some people think younger children may be able to perform like adults when crossing the street.
"Our study shows that's not necessarily the case on busy roads where traffic doesn't stop."
Crossing the street by foot seems easy for adults who take stock of the traffic and calculate the time needed to get to the other side without being hit.
But the research showed it's anything but simple for a child after placing six to 14 year-olds in a realistic simulated environment and asking them to cross one lane of a busy road multiple times.
Prof Plumert said parents should take extra precautions by being aware their child may not be able to judge if a traffic gap is large enough to cross safely.
Young children also may not have developed the fine motor skills to step into the street the moment a car has passed - like adults have mastered.
And a child may allow eagerness to outweigh reason when judging the best time to cross a busy street.
Prof Plumert said: "They get the pressure of not wanting to wait combined with these less-mature abilities. And that's what makes it a risky situation."
About 1,500 pedestrians under the age of 15 are killed or seriously injured on Britain's roads each year.
In the US the National Centre for Statistics and Analysis reported 8,000 injuries and 207 fatalities involving motor vehicles and pedestrians age 14 and younger in 2014.
Prof Plumert and colleagues recruited six, eight, ten, 12 and 14 year-olds as well as a control group of adults.
Each participant faced a string of approaching virtual vehicles travelling 25 mph -considered a benchmark speed for a residential neighborhood - and then crossed a single lane of traffic about nine feet wide.
The time between vehicles ranged from two to five seconds. Each participant negotiated a road crossing 20 times for about 2,000 total trips involving the age groups.
The crossings took place in an immersive, 3-D interactive space at the Hank Virtual Environments Lab on the university campus.
Study co-author Elizabeth O'Neal, a graduate student, said the simulated environment is "very compelling." She said: "We often had kids reach out and try to touch the cars."
The six year-olds were struck by vehicles eight percent of the time, eight year-olds six percent, ten year-olds five percent and 12 year-olds two percent. Those aged 14 and older had no accidents.
Ms O'Neal said children contend with two main variables when deciding whether it's safe to cross a street.
The first involves their perceptual ability - or how they judge the gap between a passing car and an oncoming vehicle - taking into account its speed and distance from the crossing.
Younger children had more difficulty making consistently accurate perceptual decisions.
The second problem was their motor skills - how quickly do they time their step from the kerb into the street after a car just passed.
Younger children were incapable of being as precise as adults giving them less time to cross the street between cars.
Ms O'Neal said: "Most kids choose similar size gaps - between the passing car and oncoming vehicle - as adults. But they're not able to time their movement into traffic as well as adults can."
The researchers found children as young as six crossed the street as quickly as adults - eliminating speed as a possible cause for their vulnerability.
One recommendation is for parents to teach their children to be patient and to encourage younger ones to choose gaps that are even larger than those adults would choose for themselves.
Ms O'Neal also advised civic planners to identify places where children are likely to cross streets and make sure those intersections have a pedestrian-crossing aid.
Prof Plumert said: "If there are places where kids are highly likely to cross the road, because it's the most efficient route to school, for example, and traffic doesn't stop there, it would be wise to have cross-walks."