Manturzewska (1990) in a study of 135 professional musicians over their lifespan, found that the most common age to begin instrumental music lessons was between 5 and 6 years. This is when the majority of the professional musicians began their musical instruction. Those who started late, after 9 years old, were less likely to reach virtuosic capabilities regardless of musical abilities and degree of motivation. Latecomers can reach international artistic competence, but this was only found for conductors and composers.
Manturzewska, M. (1990). A biographical study of the life-span development of professional musicians. Psychology of Music, 18(2), 112-139.