As far as "changing the world" goes; the reality is you mostly have to be very lucky. The amount of people setting out to do world changing things vs. the ones that are successful is a staggering ratio.

Hitting 40 this year my advice to any young person is do what will make you feel happy and fulfilled. Whatever that is. It's hard to figure that out, but we only live once and the old adage "You'll regret the things you didn't do more than the things you did." is generally true.

The one sentence version is; say "Yes!" to more things.