Matthew McConaughey's mother, Kay McConaughey, says this parenting rule helped her raise three sons. L. Cohen/WireImage

Matthew McConaughey's mother, 94, says she stuck to this one non-negotiable rule when raising her three sons.

She expected her kids to follow the standards she set at home while living under her roof.

"After you turn 18, you are out of here and can do what you please," Kay McConaughey said.

Matthew McConaughey 's mother, Kay McConaughey , says there's one parenting rule she always stuck to while raising her sons.

"Be consistent," McConaughey, 94, told Southern Living in an interview published on Saturday. "Set up the rules and say 'This is it. It's not up for discussion.…