Children (3–6 years old) and adults were trained for 6 weeks to identify a single tone, C5. Test sessions, held at the end of each week, had participants identify C5 within a set of seven alternative tones. By the third week of training, identification accuracy of children 5–6 years old surpassed the accuracies of children 3–4 years old and adults. Combined with an analysis of perceptual strategies, the data provide strong support for a critical period for absolute pitch acquisition.
Russo, Frank A.; Windell, Deborah L.; and Cuddy, Lola L., "Learning the Special Note: Evidence for a critical period for absolute pitch acquisition" (2003). Psychology Publications and Research. Paper 4.