If you plan the rest of your life out while you're still in your teens, you'll likely end up with a bad plan (because you lack the life experience to know what a good plan looks like), and you will probably miss out on a lot of the nicer things in life. There's something to be said for stepping back and experiencing life now while you're still young and flexible instead of waiting until after you've achieved "success" and are consequentially old and creaky.

Plan = What to accomplish in your life