Are teachers, accountants, mathematicians, scientists, doctors, or lawyers known for practicing to get into their respective fields in elementary & middle school? Not so much. Even for the most demanding fields, we expect them to begin to specialize only in their high school years.
To my mind, the fields which are known for requiring training from childhood are sports and the performing arts (music, theatre).
And if I had to guess, I'd speculate this is less because it's intensely difficult to play sports or learn to play music, but because so few people can become professionally successful in these fields (there's only room for so many sports/music/film stars). CS doesn't have this problem (the demand for programmers is skyrocketing), so we shouldn't expect folks who learned to program in childhood to have any more than a mild-to-modest advantage.