Career Goals During High School

At what age do teenagers need to have a good idea of what they "think" they want to do, in terms of a career, and start planning it as far as what classes to take in high school and college?  My daughter is a freshman so she has a few years yet and I know my stepdaughter changed her career goals the last year of high school.  Many kids don't make up their mind until after high school, right?

My oldest used to want to be a vet, a nurse (though I can't see her shopping for nurse scrub sets at http://www.blueskyscrubs.com if the way she squealed when I cut my finger last week was any indication), an artist, a horse trainer, an architect, and now interior designer.  She is 14 - she has time to get it figured out.

As an adult, I would change my mind if we could afford it - I'd go to law school!  At the very least, I'd finish my paralegal degree but becoming an attorney would be my dream.