Adult means same shit, different day. The world has not changed so much, only your part in it. What that part is? Well that depends on what you intend to do. Think about philosophy, not necessarily finding one and sticking to it, but finding as many as you can and being aware of them, whether they have names or not, these are the ideas - the people, who surround you.
Your power, which is greater than you will know when you are younger, must be curbed to fit within the cohesive structures of law and order, to allow tribes - the size of which we cannot individually grasp - to function (well?). 18 is an arbitrary marker, it does not mean you suddenly cease to be a child, just "Surely they should have gotten the hang of things by now."
You know what a decade feels like - it's been neatly segmented for you. Whether it feels like everything or nothing, you have more of them to experience. Keep them in mind and wonder what you might feel like doing with them, John Lennon said; "Whatever gets your through the night, it's alright."
I hope that helps.