I think you can know what your strengths are at a young age and know what careers might suit you as a result. I am sure we all know or knew someone who is a born teacher, farmer, doctor, nurse etc. Not hard and fast and I doubt two people working in the same career have the same experience in the workplace, but I think some kids do know whether they love the education environment, the healthcare one (I have spent plenty of time in urgent care with mine and various follow ups!), being with animals etc. I don't think many people know they want to grow up to be an accountant or software developer, but those that do end up in those fields probably could identify parts of their mind and aptitudes that are there from a young age. I guess the good news is that careers are dynamic now, and plenty of people can find variety in a field or pivot to an entirely different one.