Final answer:
The brain's nerve endings continue to grow until at least early adulthood, with significant development, including the full development of the frontal lobes, occurring by 25 years old.
Step-by-step explanation:
The number and size of the brain's nerve endings continue to grow and develop significantly during adolescence. This process is part of a larger phenomenon of brain development that occurs during this stage of life. Cognitive development and increased brain efficiency are facilitated by processes such as synaptic pruning and myelination, which enhance the brain's functionality. Accordingly, the frontal lobes become fully developed by 25 years old, indicating that the brain's growth in terms of structure and cognitive capacity continues at least until early adulthood.