I'm still working on the answer to that question, and I've been "grown up" for several decades now. :-)
Seriously, for some people I think there may not be a final answer to that question. What you want to be when you are 15 may change by the time you are 30. It could change several times over the course of a lifetime. People grow, learn, and change, and so might their idea of what they want to be.
There's no rule that says you have to choose one thing, and then be that thing for the rest of your life. For some people it works out that way, and they are quite happy, and that's great. For me, and I think for many others, a better question is: "what do I want this part of my life to be about?", realizing that I am not locked into my answer forever.