It was long thought that neurogenesis (the generation of new neurons) only occurs during embryonic development and ceases postnatally in the mammalian brain.
However, over the years many studies have reported adult neurogenesis occurs in certain parts of the brain (e.g. a consistent rate of neurogenesis occurred in the human hippocampus well into the 90s) and so attention has since shifted towards understanding the mechanisms by which new neurons are generated in a mature brain and the role of adult neurogenesis.
What is particularly interesting is the finding that environmental experiences such as learning, exercise, enriched environmental stimulation, and stress can have marked effects on various aspects of adult neurogenesis e.g. rate and extent of neurogenesis.