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1.
I phrase used by young people, similar to yolo who enjoy living and want to do everything that they can when they have the chance instead of missing the opportunity and behaving like old/middle aged people when they could be having fun.
If this is properly followed it leads to the most memorable nights in your life.
Jack: I'm going to a party
Jack's mum: Why?
Jack: Cause I'm young

Tom: Dude why the hell are doing this?
Jack: Cause I'm young
by Bandit, the Fearless Ferret December 23, 2012