That's a question a child frequently asks. Until recently, the question was understood to be a license for unbounded dreaming.
Some folks keep asking the question of themselves throughout life.
The architects of STW deem it socially inefficient to permit indecision through the teen years. Seeded with government money, a huge industry has sprung up in directed careerism. Then there's the branch of the STW cottage industry called Vocational Biographies. The stories now comprise a 35-volume library that STW facilitators can use to get children thinking very specifically about what they want to be when they grow up. Yet, one must concede this would make dandy supplementary material for a unit in Death Education, in keeping with progressive education's infatuation with the desensitizing macabre.