It is well known that the young mind is especially attuned to absorbing copious amounts of knowledge and learning skills that will last a lifetime. Skills like riding a bicycle or swimming are not readily forgotten. Furthermore, if a child is receptive, they will learn skills like playing the piano much more readily than an adult and will be likely to retain those skills well into adulthood. Rashi, in a comment on Talmud Bavli Shabbat 21b, identifies this as girsa de-yankuta.









