@Nasenspray I don't think it's a good question to ask: "when did other people knew what they want?" It's too heavy and it implies a somewhat "static" world: nach dem Abi hab ich realisiert dass ich bei Feuerwehr arbeiten will und dass habe ich auch biss zu meinem Tot gemacht". Does the world still work that way? One profession, you arrive and then stay there for ever? Maybe. But I find much more satisfying the prospect of being able to move "through" life. World is changing and so are we. So perhaps a better topic for the 15-years old to contemplate could be "Where do I want to start exploring what I want to achieve in this world one day?" All that with full knowledge that it's just a start and you can become many things in life. If you give yourself 15-20 years to be one thing and be good at it, you have 2-3, maybe 4 professions, careers in one life. Isn't that a cool prospect? You can be one thing and then move on as you develop and mature to become something even more interesting/satisfying/fulfilling and so on until you really find what defines you. And along the way you mature, collect experience and wisdom so that when you figure it out, you'll have a shot being really broad person and good at what you want to be your life's legacy. I really find it appealing to see people moving from corporate finance to becoming a teacher, or the other way round. From a factory floor worker to become an entrepreneur, or perhaps get into elderly care - I see people like this around me and I admire it.

My point is this: when you think about it as a start of a journey as 15-years old with an open-ended finale, it takes away a lot of that heavy weight of decision. "How do I want my life to be defined, what do I want to be?" That is a very heavy question to ponder at 15. Actually starting almost anywhere is as good as anything else. It's more important to be conscious that it's just a start and you'll be exploring and moving so as to eventually figure out what you really "want to be".