The question that is the title of this post may seem an innocent one. To some extent it is, and some hostile reflections on it which occurred to me today may seem, may be, eccentric and exaggerated. And yet I think the question expresses a way that our culture indoctrinates youth, which is false, and has a high cost. In Plato's Republic, he proposes that the citizens of his ideal state be taught to believe a "noble lie." Is our society based on a "noble lie," too? The lie I have in mind is difficult to express, but the words you can be anything you want to be might capture it.
"What do you want to be when you grow up?" The question implies (a) that a kid should have some idea, at an age of ten or eleven, of his future career, and (b) that his future career can and/or should depend on his wishes, his inclinations, his daydreams ("want"). The trouble with (a) is that the economy is so complex that a kid won't know a hundredth part of the careers that exist, and to encourage such early planning might lead to a surplus of aspirants to a handful of careers that kids know about, e.g., teachers, movie stars, soldiers, president of the United States. But of course, kids aren't making any hard-and-fast commitments when they respond to grown-ups' small talk, and some careers do require long advance planning. So (a) is all right.
No, my problem is with (b). Suppose grown-ups asked a different question: "What do you think you might be useful for, when you're grown up?" This is a more wholesome question because it doesn't invite a kid to project his self-indulgent wishes and daydreams onto his plans for the future, but, instead, invites him to think of others, of service, of how he can help. It doesn't suggest that the world owes him an interesting, exciting, or entertaining job, but that he'll owe the world valuable services in return for his room and board. It encourages him to appraise, not his desires, but his capacities.
"What do you want to be when you grow up?" The question suggests that it matters what one wants to be, that one can choose, that you can be anything you want to be. The truth is that the career paths that will open up to a person are a tiny fraction of those he can imagine. It depends partly on aptitude: that is relatively easy to understand. A tone-deaf person can't be a musician; a stupid person can't be an MIT professor. More subtly, it depends on a thousand accidents of social connections and circumstances and which resume strikes whose fancy and whether one happened to have a good day on this or that test or interview. Try your utmost to realize your childhood dreams, and you'll probably fail; you may waste much effort and sacrifice personal ties and innocent pleasures in the process; and, worse, you may be corrupted. There are a thousand subtle moral compromises to which the path to success may tempt you.
And yet, for all that, for boys, it might still be right to encourage the pursuit of dreams. It's in men's nature to take risks. Courage is stereotypically a masculine virtue, and while the stereotype is partly false-- marriage and childbirth may require as much courage from women as battle does from men-- the occasions that call for courage in women tend to be more a matter of accepting one's lot and less a matter of going out to seek adventure. Risk-taking has a basis in masculine biology-- in nature, males are more expendable than females-- and this biological factor is etherealized in men into cultural manifestations of honor and bravado. And from a rational point of view, too, men can better afford to take risks, for the simple reason that a man has more time: men usually marry younger women, and they can beget children for much longer than women can bear them. A man can pursue his dreams and fail and pick himself up and be richer for the experience.
It is in girls that I think the attitude you can be what you want to be causes the greatest unhappiness. Our society seems to be engaged in a conspiracy not to tell girls when they are young that they may have to choose between marriage and career, or if not that, at least between the dream career that strikes their fancy, with all the complex and urgent demands it will make on them, and the settled life and strong responsibilities of motherhood. Political correctness can prevent grown-ups from talking to kids about this but it can't change the stubborn facts that female desirability as a marriage partner tends to peak in the 20s, well before investments in high-powered careers have had time to pay off, and that there aren't enough hours in the day to spend 50 hours a week in the office and pick one's kids up after school.
All in all, though, I think we would do better not to encourage self-indulgent personal ambition in men or women. America is indeed a land of opportunity, but the best opportunities usually come as surprises. We should be alert for opportunities to be of use, and able to appreciate the chances that time tosses in our path. Meanwhile, the question we should focus on is not what we want to be, but what we ought to be.