Whoever started the idea for offspring to leave home at age 18 or 21 is evil genius

Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.

-- Genesis 2:24

And said, For this cause shall a man leave father and mother, and shall cleave to his wife: and they twain shall be one flesh? 

-- Matthew 19:5

For this cause shall a man leave his father and mother, and cleave to his wife; 

-- Mark 10:7

For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh.

-- Ephesians 5:31