Generally you start thinking when there's almost no time to think that's how we're designed. But ideally it should probably begin when you have enough time to weigh all the pluses and minuses of the career you're choosing. After doing that you can sort out all the things that you certainly dislike and then you can brood over the remaining options. The ideal time for that in my opinion is the eighth grade because a person can think more or less rationally at this stage.
