During the first decade of life, the cerebral cortex undergoes a dramatic curve in energy consumption. Metabolic rates in the brain rapidly increase beginning at birth and begin to reach adult values around age two. This plateau is the result of hyperconnectivity, where cortical neurons have formed excessive connections, which are later either preserved or selectively eliminated depending upon exposures and stimuli. Around age 10, plasticity of the brain begins a gradual decline until 16 or 18, at which point, the levels of glucose utilization have reached adult values. Because the regions of the brain develop systematically, there are critical windows of opportunity for learning.
Different regions become more malleable during particular phases.
Once that time is up, you can never recapture that unique ability. This concept was proven in the 70s with experiments showing that cats can be blinded simply by covering their eyes during critical periods of infancy. If the connections are impaired during the windows of opportunity, the brain will never be able to perform the task, even when the eyes are reopened. The same logic applies to humans. So if your little girl wants to play a musical instrument, she shouldn't wait until age 20 when the cortex is already developed.