New neurons stop growing in a key region of the brain's "memory centre" as early as adolescence, according to a controversial study.
Scientists looking at brain tissue samples found no evidence of new nerve cell growth in the dentate gyryus, a part of the hippocampus vital to memory formation, after the age of 13.
The discovery contradicts previous findings suggesting that hippocampal neurons replenish themselves throughout adulthood, as they do in many other mammals.
The newly found pattern seems to be a hallmark of big complex brains.
New hippocampus neuron development also dwindled over time in macaque monkeys, the scientists found. And hippocampus regeneration was thought to be lacking in big-brained dolphins, porpoises and whales.
Writing in the journal Nature, the US authors led by Dr Arturo Alvarez-Buylia, from the University of California at San Francisco, concluded: "Neurogenesis in the dentate gyrus does not continue, or is extremely rare, in adult humans.
"The early decline in hippocampal neurogenesis raises questions about how the function of the dentate gyrus differs between humans and other species in which adult hippocampal neurogenesis is preserved."
The scientists used marker antibodies to "flag up" neural precursor cells and immature neurons in 59 human brain tissue samples.
The post-mortem and post-surgery samples spanned all life stages from the foetus to older adulthood.
The number of developing neurons in the dentate gyrus reduced with age and the oldest sample in which any were found was from a 13-year-old. No young neurons were detected in samples from people aged 18 to 77.
The scientists added: "Interestingly, a lack of neurogenesis in the hippocampus has been suggested for aquatic mammals (dolphins, porpoises and whales), species known for their large brains, longevity and complex behaviour."
Previous studies may have wrongly reported the detection of immature hippocampal neurons because the marker proteins used to disclose them do not work the same way in humans and animals, said the researchers.
In an accompanying News & Views commentary, Dr Jason Snyder from the University of British Columbia, Canada, wrote: "These findings are in stark contrast to the prevailing view that human hippocampal neurogenesis extends throughout adult life."