Do you still ask for your parent's permission at 18?Watch
If you want to show your parents you're a grown up, I think it's much better to be open with them and respect their opinion, rather than take your new found adult-freedom by the horns and do all the things you know they don't like you doing.
I would think at 18 you can make your own choices, but its a matter of consideration to inform them if it might affect them.
Some parents are overprotective or they have difficulty accepting you are a grown up or there are strong cultural issues. Whilst you are still under their roof then they get to call some shots.