Researchers used MRI scans to monitor changes in the structure of youngsters' brains from childhood to early adulthood.
The study focused on white matter, a key tissue which includes the axons - brain cell elongations that carry signals between different brain areas.
The development of white matter in young people is closely associated with gains in cognitive function - or brain power - prompting the researchers to wonder if there could be differences between males and females.
The team at Pittsburgh University carried out annual brain scans over five years on 128 people, initially aged 8 to 28, to observe what changes took place.
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"Males showed continuous white matter growth from childhood through early adulthood, whereas females mainly showed growth during mid-adolescence," researchers said.
"Further, earlier white matter growth in adolescence was associated with faster and more efficient responding and better inhibitory control," they said.
The researchers suggest the difference is linked to girls experiencing earlier puberty.