Perhaps the better question is; "Why do you want to grow up?"!
Remember a time in the dim mists of time when life's riddles seemed simpler? Grown ups would delight in endlessly interrogating any young child in sight as to what they wanted to be when they grow up. Answers such as doctor, nurse, pilot, builder, policeman were positively and knowingly approved. Ask the same question today and you might feel less enlightened by the answer.
Traditional jobs are 'all so yesterday'. Much in the manner that a previous generation may have responded to the career choice perhaps of a cobbler, chimney sweep or basket weaver. It's all in the realm of black and white movies of a bygone era when politeness and formality ruled over spontaneity and individualism.
Predictability in our warp drive journey through the vast digital world of untapped knowledge is all but dead. 'What do you want to be when you grow up?' Is a question that presumes some future Nostradamus-like insight into the shape of things to come.
The modern child prodigy is more likely to define their future career goals in terms of what they want to become rather than how they want to spend their lives. Predictability is the antithesis of the new generation's future strivings. It's all about now. Predicting future trends is the new Lotto. It's all about finding contemporary niches to manipulate. 'Niche' is the new buzz word for the ambitious. It's not about becoming an engineer or accountant anymore.
It's about finding and tapping into a hidden somewhat obscure yet lucrative market place. Perhaps garden gnomes. Find a community of gnome enthusiasts and cater to their predilections Join their social media communities. Blog with them. Identify their desires and provide solutions. Sell them an e-book on some curious angle about their passion and conquer the world!
The glazed somewhat confused look your question may elicit may not be the result of lack of ambition, but perhaps the instinct that their answer will be so incomprehensible to you that it's not worth the effort answering.
It's a new world where spontaneity, laced with a touch of hedonism and living in the moment, whilst not particularly sensible and definitely not predictable is undeniably perfectly adaptable. Adaptation is the key to survival in any environment and perhaps this new unblinkered approach to career possibility is enlightened.
Just don't expect a simple answer to your question!