A study involving 70 older adults (60-83) has found that those with at least ten years of musical training performed the best on cognitive tests, followed by those with one to nine years of musical study, with those with no musical training trailing the field.
All the musicians were amateurs who began playing an instrument at about 10 years of age. Half of the high-level musicians still played an instrument at the time of the study, but they didn't perform better on the cognitive tests than the other advanced musicians who had stopped playing years earlier. Previous research suggests that both years of musical participation and age of acquisition are critical.
All the participants had similar levels of education and fitness. The cognitive tests related to visuospatial memory, naming objects and executive function.
Hanna-Pladdy, B. & MacKay, A. 2011. The relation between instrumental musical activity and cognitive aging. Neuropsychology, 25 (3), 378-86. doi: 10.1037/a0021895