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8.
An adult who never grew up. Not necessarily a bad person, but someone who doesn't have motivation to achieve independence outside of his parent's domain.
Usually a kidult is characterized by living in their parent's basement (typically male), have an inability to find a job and usually addicted to video games.
by Sue Danim April 22, 2009
 
9.
A person in transition between childhood and adulthood.
"Do you have any ID, you look a little to young to be an adult"
by max May 16, 2005