The study, carried out on mice, also showed that in response to a surge in thyroid hormone, heart muscle cells undergo an intense 24 hour "burst" of division in preadolescence.
During this burst, the number of heart muscle cells increase by more than 40 per cent, or half a million cells, and compared with later in development, the ability of the heart to recover after injury was enhanced.
This response is essential for the heart to meet the increased circulatory needs of the body during a period of rapid growth in preadolescence, in which the heart increases almost four-fold in size.
In a study published in Cell, the scientists have overturned more than a century of scientific theory, which proposed that heart muscle cells in mammals stopped replicating just after birth, limiting the organ's ability to repair itself after injury.
The findings suggest that thyroid hormone therapy could stimulate the process, and may even enhance the heart's ability to regenerate in patients with heart disease.
Bob Graham, Executive Director at the Victor Chang Cardiac Research Institute and Des Renford of Medicine at UNSW, with Ahsan Husain at Emory University, led the study.
"Heart muscle cells retain the ability to divide and make new cells for a long time after birth, at least until preadolescence, equivalent to eight to ten years of age in humans," Graham said.
"The implications of our findings could be huge, as it may give us a significant window of opportunity in which to repair the hearts of babies born with heart defects, or even to reactivate heart muscle cells damaged after a heart attack in adults," he said.
"The scientists also believed the brevity of the burst may explain why it has previously gone undetected, taking place over just 24 hours in mice, equating to around five weeks in humans," he said.
"I think this research has given us some really important and significant insights, including that the heart is not as static as we previously thought," Graham added.