To "fly the coop" either means to grow old enough to leave the house of one's parents or in the state of being far to cool or above the current predicament that one find themselves in and find it necessary to leave immediately.
Johnny had discovered that he was old enough to fly the coop and move away from his parents.

Anna decided to fly the coop and leave the party, no one of her high social status had decided to show up.
by mhmmmmmmmm. December 18, 2011
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