According to a study conducted by the University of Iowa in the USA, children under the age of 14 should not be allowed to cross the street unassisted. This conclusion was reached after placing kids from the ages of 6 to 14 in a simulated environment to recreate an actual road to understand how they react to passing cars as well as how soon after a car has passed do they take action and cross the road. This allowed researchers to assess how they perform by attempting to cross one lane of a busy street multiple times.
As it turns out, up until their early teens, kids are unable to consistently cross the lane safely due to underdeveloped fine motor skills that have the function of allowing them to identify gaps that are long enough to take advantage of. Add to this the fact that children are impatient and may cross the lane in an untimely manner has led to thousands of injuries and hundreds of fatalities in the USA alone almost every year.
There is a chance of risk that starts high at age 6 and decreases progressively once kids reach age 14 at which point it is safe for them to navigate busy streets unaided. The thorough study where each kid crossed 20 times for a collective amount of 2000 trips across these ages leads to the conclusion that no matter how we as parents feel that our younger children are capable and aware to cross a lane by themselves, it is statistically better and safer to refrain from letting them do so for their own safety.