By what age should you know what you want to do with your life?
Most adults don't know what they want to do in life...But you should pick some kind if career path at an early age regardless if that is what you really want to do, you can always change if something comes along
things like DOCTOR and that pretty early . you can always change it .
As young as possible
Probably 13-16 is a great age to start thinking about that sort of stuff.