Becoming An Actress..what needs to happen?

How does most ACTORS and ACTRESSES get their start? Is it their calling? Is it because a family member of their's is alreadya famous actor? Is is because it runs through their blood and they know they got it? I mean what is it? And is there really an age limit into getting into Hollywood? I don't think there is. If you want act? what should you do? Just go for it? I mean look @ Will Smith, Denzel Washington, Mat Damon, Mark Whalberg, Nicole Kidman, Catherine Zeta Jones, Lenardo Dicaprio, Heath Ledger, Brad Pitt, Angelina Jolie, Halle Berry, Drew Berrymore..and more.