these phrases are not to justify your stupidity. 
honestly, I can’t say how much of a screw up younger generations are becoming.
You Only Live Once doesn’t mean you go underage drinking, having casual sex, smoking, doing drugs and all that shit. does not give you “swag”. It doesn’t give you a right to do WHAT EVER you want. Some things are not meant for you to try. 
Fuck It I’m Young doesn’t mean you go disrespecting elders, doesn’t mean you let go of your hopes and plans for the future to concentrate on the things that’s happening “NOW” in your life. If you say to everything “fuck it, i’m young” well, one day, you’re going to grow old. Your future will reflect upon the choices you make when you’re young. 

SO, before you go “YOLO” or “FUCK IT I’m YOUNG” to anything that regards your life, think twice about the choices you make. Because, honestly, you’ll go downhill with that attitude.